"Fortitude" refers to the ability to endure, not just hardship, but the sense of being secure and of one's actions being long lasting, and not momentary, trivial or short lasting. If one has "Fortitude," one is not necessarily the strongest person, and that is why competitive qualities such as "strength" or "toughness" or "energy" or "zeal" are not gifts of the Holy Spirit, while "Fortitude" is. To fully understand one must think of the nature of life in Biblical times, and also the relationship that humans have with God, and express that relationship in the lingo of those times.
In order to be safe during those very dangerous and lawless times, humans gathered themselves together under the protection of a fort. The defensive protective building structure-a fort-is where the concept and the word "Fortitude" comes from. Forts took one of two forms. Either the city itself was a fort because it had a protective wall with strong gates for entry and towers for observation and defense, or a fort was nearby, dedicated to being a refuge in siege and the place where military men served to observe and protect not only the nearby city but also key roads and routes of travel. This is why the tone of the gift of "Fortitude" is one of unflappability, constancy and reliability. The fort is always there and is watchful, since being alert and on guard is as important as one's skill in actual defensive battle. One can think of the fort as being like what we call today "homeland security." A person who has the gift of "Fortitude" is like a person who serves as either their own and their family's self-contained "fort of refuge" or a person who is secure in knowing that a fort of refuge is nearby.
Thus if one reads the Bible one does not see reference to the attribute of "Fortitude," but instead one sees the praise of God who is the fortress of the faithful. This is why one discerns from studying the Bible that a gift of God through the Holy Spirit is "Fortitude," both one's own, having developed from the foundation of the gifts of "Fear of the Lord," "Piety," and "Knowledge," and that of God, knowing that God is there, God is nearby, and God is one's fortress.
2 Samuel 22:1-2, 40
David sang the words of this song to the Lord when the Lord had rescued him from the grasp of all his enemies and from the hand of Saul. This is what he sang:
"O Lord, my rock, my fortress, my deliverer, my God, my rock of refuge...the foreigners fawned and cringed before me; they staggered forth from their fortresses..."
King David starts with the image of God as his fortress in the very first line of his song of thanks and praise. Forty lines later he returns to that specific image, where the invaders were defeated out of their own fortresses through God's aid to the might of King David's defending soldiers (in fact the names of his most outstanding commanders are preserved, along with their deeds, in 2 Samuel 23:8-39. I mention this to constantly, gently demonstrate to you that the Bible is a work of historical accuracy in addition to being a book of faith, and you can see that by the great detail that is recorded of the names of those who do not have an ongoing part in some "mythology," but are instead, people who said and did actual true life things). Because this is so important and here is a great example, let me make an aside to cite just one warrior so you can witness how this is a battle list of actual people, and not just part of some mythological writing.
2 Samuel 23:20-23
Benaiah, son of Jehoiada, a stalwart from Kabzeel, was a man of great achievements. It was he who slew the two lions in Moab. He also went down and killed the lion in the cistern at the time of the snow. It as he, too, who slew an Egyptian of large stature. Although the Egyptian was armed with a spear, he went against him with a club and wrested the spear from the Egyptian's hand, then killed him with his own spear. Such were the deeds performed by Benaiah, son of Jehoiada. He was listed among the Thirty warriors and commanded greater respect than the Thirty. However, he did not attain to the Three. David put him in command of his bodyguard.
We are not going too far astray from the topic of "Fortitude" in looking at the historic example of Benaiah, son of Jehoiada, because if you read it carefully it gives you crucial insight into the hardship of the times, and also the idea of the defensive fort. Benaiah clearly had reknown as having done a famous deed of slaying two lions in Moab. Now, animal lovers, don't get excited. Lions were numerous and a threat to humans, often preying on women and children as they had to gather food or go to the well (cistern) for water. When the Bible uses the word "slay," this implies a conflict, rather than use the word "kill," which may or may not be a justifiable death. So one can deduce with confidence that Benaiah was already famous for having slaying two predatory lions in Moab, and also going down into the cistern, where a lion was hiding during the snow, and slaying it, thus protecting the populace. Then it is recorded specifics of his defeat of a large and better armed Egyptian, and the specifics of his grouping in the elite unit of Thirty. There is even commentary that he was better than most of the Thirty, but did not make it into the separate better group of the Three! Yet David selected him to be his bodyguard, and I'll bet it's because of the lions too! One of the points I am trying to make is that Benaiah, son of Jehoiada, became David's bodyguard because of his protectiveness in addition to his prowess.
The gift of "Fortitude" must therefore be understood as being not only self contained and secure, but also protective of others. One with "Fortitude" is responsible and endures not just for one's self, but on behalf of others. It is not like the popular phrase of today "I am being strong for their behalf," when someone, let's say a grieving family member is "holding it together" so that others don't see him or her cry. That is being "strong," that is not having the gift of "Fortitude." "Fortitude" is when one does not have to mask or pretend because, instead, that person is genuinely serene in the face of even the greatest burden, so great is their faith in God and thus their own capabilities and capacity. This person also consistently maintains their perspective and context about life, what the big issues are and the little ones, and deals with them accordingly and, importantly, they never forget the comparative context of both human life and eternal life.
