I understand that it is hard for humans to understand this. Humans tend to think, well, if God "knows" in advance everything that is going to happen to a new person, if it is a "bad" and "hard" life, won't he send a "stronger" soul, or make some sort of other discrimination about which soul to which body? No, absolutely not, because that is a human mindset, not God's. The only time a soul was ever "assigned" to a body in anticipation of earthly events was obviously Jesus Christ, who was fathered and whose soul was provided by the "overshadowing of the Holy Spirit." That should be obvious to anyone. This is another way for those who puzzle over God having "a Son" to understand it. Jesus was the only one who was given his soul as part of the pre-existing plan by God and you know that because of the giving of his life (and obviously soul) by the actual indwelling of the Holy Spirit. All other human beings receive a new, clean and equal soul at the time of their conception and quickening (first life impulse) in the mother's womb, one that is not "matched up" with what God "knows" will happen, but one that is individually created through God's grace.
How to help you better understand this from "God's point of view?" Well, think about angels. There are uncountable numbers of angels, yet each one is individual. They are individual even though they don't really "do anything" easy or hard, except dwell in the presence of God and for a few, manifest his will among humans. So each individual angel is absolutely unique even though they do not have individual corporal bodies and in fact humans cannot tell them apart unless they self identify. For example the four cherubims that appear to Ezechiel appear identical to him (Ezechiel 1:5-7). Yet obviously God created them individually and "can tell them apart." It is like the humor we have about some of God's animal species here on earth where a human can't tell male or female of the species, and the punchline to the joke is that "Well, they can tell the difference!" :-)
So God being God, obviously, creates and perceives an uncountable number of his creatures, in this case angels, that are equally loved and yet unique in his eyes. It is therefore using this analogy easy to see that at the time an embryo is conceived that God creates one of his individual but equal clean souls to send, just as he did the uncountable number of angels.
Think of all the preborn babies who have died. I do not mean to be sad here or to raise the topic of abortion, so just think of it as the miscarriages of babies that have occurred throughout human history. Each baby received a unique and equal soul, and none of them lived an actual life since they died before birth. Yet they each had a unique soul and they each go to heaven with their one soul that has been shaped by their experience of being alive, no matter how brief, since they resided, even very briefly, in their body, which is the temple of the soul. Suppose the baby was in a mother who experienced deprivation and starvation, and that was the reason she miscarried? The pre born child shared with her the experience of not having enough calories, of having to perhaps travel long miles for a handful of food. The baby did not suffer consciously in this case, and so the baby was serene, even as it did not thrive due to lack of calories and died. Yet as a soul the preborn baby shared the hardship of being in a body that could not thrive and did die. Therefore when the soul goes to heaven (as all infants do) the soul will have been shaped in a pattern that has some of the glory of martyrdom and sacrifice, and will join with members of the family already there (grandparents for example) in a way that is a full member of the family and with that "orientation." In other words, even a few months of life as a pre-born baby gives a dignity and individuality of life that is equal, within heaven, to someone who lived for a hundred years.
So this illustrates the second part of God's individual plan for each person which is equality in love and equality in soul "maturity" and potential access to heaven regardless of the actual length of the human's life. There is no such thing as an "old soul" or a "young soul." All souls are the same "age," whether the human body lived for a few months as a pre-born baby or a full life span of one hundred and twenty years or more. A pre-born baby who died from starvation due to its mother's deprived calories as in the example I give up above is as "old" in soul as a person who died after a full life and achieved heaven. To help you to understand that, think of a soul's "age" as not being about time (which does not exist in heaven), but about virtue. A baby who shared his or her mother's suffering and lost their life as a result is as full of virtue as a human who, on and off, pursued a good and worthy life and even with sins, repents and finds heaven. A soul's "age" is not about time frames or "lessons," but the amount and type of individuality and virtue the soul lived, however short the time.
And that illustrates the third part of God's individual plan for each person which is that regardless of what happens to that person when they are alive, the potential for God to find the good and to steer them in that direction is consistent for everyone, but obviously varies according to their environment of birth. A pre-born baby that starves in the Sudan, for example, is dependent on the limited earthly options offered to the mother. It's not like if the mother walked long enough that she could reach a grocery store and find a wallet full of money. So God's "plan" for that pre-born baby is not that he or she should starve and be miscarried. God's "plan" for that baby is that 1) the baby obviously is a child of God's and will achieve paradise and 2) the Holy Spirit will continually move to try to bring people with grace to bear on the situation so that some good can come from even corners of the world that suffer.
So, for example, a fourth part of God's individual plan for each person is to promote charity and goodness in the surrounding individuals so that no matter where one is on earth there is no total absence of goodness and hope. It may not be enough to result in a long life, but it is part of that individual's "plan" by God that agencies, governments, individuals and religious groups feel the stirring of both brotherly and sisterly love AND God's love so that they try to bring peace, they try to bring medicine, they try to bring food. In a way, the starving pre-born baby, even if never individually known about, is like a glowing light that tries to pull goodness toward him or her, by the calling of people who care who try to remediate the larger situation of hunger, poverty, refugee status, violence victim, and war. So in both a spiritual way and in a bodily way, God's "plan" for that pre-born child who does die of starvation is that no life is wasted and that even the greatest sadness attempts to bias other humans toward the performing of greater good, if for no other reason than "this type of thing should not happen again." While humans are very numbers driven, God is not. For example, every child who dies of starvation has "equal credit" and "soul maturity and virtue" in heaven for resultant good deeds that develop from their hardship.
So think it through along with me. Humans tend to think in terms of receiving a report with an outrageous number of starvations, which they call a "humanitarian crisis." This will focus awareness and initiate intervention programs. There are also humans who have a "local" focus, who strive to eliminate starvation and hunger in their own village, for example, or on a local scale in a needy place of the world. So let us suppose that the pre-born child who is miscarried due to starvation of the mother is one of thousands that occur in a given place and time. Let us also suppose that at some point agencies, governments and individual food programs say, "Enough is enough" and they get some relief to the area, although obviously months or even years after our example pre-born child has perished. Obviously the people who made the intervention exhibit virtue and gain grace because as Jesus said, "he" was hungry and they "fed" him, through the poor. But you need to look at the "flip side" of what has happened to understand how God perceives humans and their souls. That pre-born baby "receives" "equal virtue" and "credit" as if he or she was the very child that was the last number of deaths that made the agencies etc. say, "Enough is enough" and send the aid. Every child who dies of starvation, for example, is honored in heaven as if he or she is the one whose death brought the saving food for everyone else after them.
Think of a modern day adult example, such as a pious person who dies of cancer. The suffering ill person may have prayed and believed, and their family may have prayed and believed, yet still they die. This is because it is the natural way of things where there are illnesses and failures of body, and it is up to humans to be charitable (and earn a living, there is nothing wrong with that) in researching cures, or better lifestyle practices and choices, or working on cleaner environments with less cancer causing chemicals and compounds. And someone says to the grieving family, "It is God's plan." Now you can see with discernment how it is God's plan, but not in the self hurting way that many humans perceive "God's will."
It is not "God's plan" or "God's will" that the person die of that disease and at that time in life, but it is God's will that humans do not live forever and that humans have resources to allocate according to their ability and charity to solve medical and other societal problems. However, one can look at the flip side of the example to understand that "God's plan" for that person is that if all else is equal (the person is a believer who lived a good life and sincerely repented of sin) that the person who dies of cancer has the credit and virtue in heaven of having been "the one" who stimulated cancer research, cancer palliative remedies, cancer home aides, stimulus to clean the environment if there is an associated cancer risk, etc. God's "plan" is to turn even the worst and saddest hardship that a person experiences into the maximum good, in ways that humans cannot see or fully comprehend (though I obviously hope that you understand more as a result of reading these two postings!) The person's soul will be shaped by their cancer experience so that they have the virtue just as if theirs was the "enough is enough" case of cancer that causes all of the good things to be stimulated as a result of the desire to mitigate the disease and condition.
See, one does not have to be a "superhero" or a "suffering martyr" to achieve heaven and to be honored there. Likewise one does not have to live long lives of super virtue and numerous charitable deeds to have a soul worthy of heaven (and in fact, works without grace is not the ticket). Soup kitchens without honoring God and having a prayer life, for example, is problematic. But this is why the average Joe or Mary has the easy road to heaven. If they live good lives of believers they arrive in heaven with the virtue of say a Mother Theresa. Suppose the average Joe lives a good life and dies of cancer. Like I said, Joe is viewed in heaven as one of the warm flames of inspiration that draws out of others around him on earth and after his death to do good deeds in the name of mitigating the cancer disease in others, even if no one ever knew Joe existed outside of his family. A guy who has a big breakthrough in cancer is "inspired" to do so by "Joe," even if Joe died years ago and he never knew him outside of being one in a table of annual cancer death statistics for research. That is because God "credits" the soul of Joe with the virtue of having the condition that caused him to suffer or lose his life, but stimulated charity and goodness (and as I said, economic stimulus) to be directed toward the problem of cancer.
So God's plan for each person is indeed individual, but it is not a checklist of things the person "must" experience. God knows in advance what will happen, obviously, because God has created the universe, and knows all and sees all. But the new person is conceived because of human free will (we all know the biology behind that), not because God has a checklist saying, "OK, it's now time for this person to pop into being!' When humans conceive according to free will and since it is God's will that humans be fruitful and pro-life, God gives the embryo a soul and the guardian angel, and the individual soul now has "a plan." That plan is to help the person navigate through the many, or the few, choices in life offered to him or her, not to rack up accomplishments or EITHER grace or works. Like I said, the pre-born infant who dies of starvation has the grace, honor and virtue as if he or she had personally caused the "inventor of the way to end hunger" to achieve their goal! So how can a human think, "Well, if I volunteer at 100 soup kitchens I am closer to heaven and better off than that other person over there?" While God exhorts humans to be good and to do good, they cannot begin to "keep score" or make assumptions about how the soul is viewed by God. And when hardship occurs, humans can only have human and neighborly charity toward their fellow human; they cannot begin to answer the "why" and the "how will they be judged" questions.
Generations of humans understood this, though obviously not to the detail that I've explained through this analogy. But modern humans are all caught up in their own double thinking complexity that is often entirely wrong. Why has this happened? First of all it's because modern humans have become so detached from the glory of normal life. It is less and less easy for people to have the simple desires that their ancestors had, for a spouse, a place to dwell, water, food and children. So as life has become more sophisticated, humans have become more unrealistic in their views of themselves, and thus see life itself as some weird and complex puzzle of risk and reward, forgetting entirely that life itself really is as simple in its foundational reality as it always has been. The second problem is that modern humans have much less faith in God, and so they become paranoid and restrict in their minds the possibilities of how good God really is and how God actually might, you know, being all powerful and everything, might actually see things differently than humans, less numerically, less scoring, less debits and credits and certainly more lovingly. Being loving does not mean telling humans that sin is "OK." Being loving means that God does everything he can for humans to understand his realness, his love and his mercy and forgiveness, and most of all, that God's love is for everyone, not segregated by human types of imaginings. So, for example, as I've shown you, even a life that is cut short in the womb can and does have individuality and virtue when the soul arrives in heaven that often is more than a "do gooder" who had a long check list of hundreds of charitable activities, but does it without love and understanding of God in his or her heart. Humans cannot even begin to understand God's goodness and how truly easy it is to achieve heaven if one only resists chronic sin and who works for the humble greater good, not the self glorying or "butt covering" greater good.
I hope this also helps you to better understand that not every little thought that enters your head and every deed that you do is prompted by God's "plan." Just like the guy going to work and stopping at the stop sign, you have a list of to do things, but you have both conscious and unconscious life plans. You may turn to God to ask what is his will in, for example, vocations, or a relationship. But remember, the first thoughts that pop in your mind are your own assumptions, both desires and fears (and random dopey thoughts that people have). Even when praying, you are still a human and your own noise can sound profound, but it is not. The wise saints knew that one must be open to God's will not so much through hoping for a "message," but by prayerful living and getting on with it. You cannot know all the implications of every decision, and the more you try the more you become astray from truly and actually understanding God. God knows what you will choose and do, but that does not mean he is "telling you" what to choose or to do. As I demonstrated, even the pre-born child who has no choice in action at all accrues virtue and individuality of their soul. Adults humans tend to be too achievement, yes/no and action oriented to actually be quiet (as in serene and open) enough to trust that God will help them to do the best that they can with whatever they choose to do (assuming it's not a sin OF COURSE!) Though God will still try to help mitigate and give chances to the person to clean up their act. God does not "send lessons" or "instructions," God travels along with you in your journey and the Holy Spirit continually influences all humans to bias toward goodness and faith.
I hope this helps!