Fortitude is the persistance in an act or way of thinking despite concrete obstacles, and belief is the acceptance as fact principles or ideas which are not guaranteed or verified by concrete evidence. Atheists even must admit that, within the human psychology, we must believe in principles that are beyond our ability to test. These principles may be as simple and earthen as transcendental-like things like good, success, glory. For the Christian, that transcendant principle becomes embodied in a man - Christ, God, on the Cross - as well as being embodied in all of othe other transcendant principles. The Eucharist translates into these things not only in tradition but in this relation of a Man to them - Christ died, embodying sacrifice, love, suffering for something greater. The essence of Christian fortitude is this sacrifice, and all that eminates from it. When it is hard for us to mentally focus on this particular sacrifice and instance of human history that coincides perpetually with our present, we may mentally access lesser transcendental examples of fortitude as well as the philosophcial isolation of these principles themselves, knowing they can never be fully isolated from the higher principles that govern the Christological mystery.
Fortitude, like all the pillars of faith, connects the concrete and material goods we seeks to access to the higher heavenly goods. For the man struggling through trial, fortitude translates the eternal principles of Christ's suffering, ministry, death and persistence in love to concrete actions and situations such as counting days, building habits, balancing schedules, and dealing with fatigue or mental exhaustion. The greatest thing about God's connection to all these things is that He as a person also has the ability to provide Joy to all of them, which means that all these exertions of fortitude involve at least perpetual satsifaction, at most the emotion of happiness.
Forgiveness in the face of failing at fortitude is especially forgiven, for Christ himself recognized the frailty of the flesh of his disciples in the Garden of Gethsemane. Many times, falling in fortitude is something outside of our control, when situations make our best exertions too difficult. This is also a paradox, for exertion when there are all concrete odds against you is a particular strength found within fortitude.
Ultimately, fortitude is taught itself by habit. Especially as Americans, we grow up in a bubble-wrapped comfort filled with goods and gifts. We are heavily blessed in our peace within comfort, a good that should be sought, but are deprived in our lack of suffering. When the comfort-filled soul meets suffering, he or she learns that ultimately we all face suffering despite all the comforts we throw up at it. To this fact it has a fight, flight, or acceptance reaction. If we fight such a reaction, we will always seek to find comfort and never face a suffering. If we flee the suffering, we will do a similar thing - throw things, not comforts, in the way of such suffering so that we can rest in our comforts and not be disturbed. Acceptance of this fact, however, allows us to commune with that necessary suffering. If we do so, then we will be prepared for all suffering, strong until any end.