Just read a reference to the Stockdale Paradox. James Stockdale was the highest ranking officer in the infamous "Hanoi Hilton" during the Vietnam War. He served 7.5 years, organized resistance among the American prisoners, thwarted efforts to be used as propaganda, etc.

It just happens he found this sustaining philosophy during a War. I'm not a fan of war, especially this one, but there's something to learn from everything. That said, here goes.

He managed using Stoic philosophy (which I mostly know through Seneca's writings). Picking up now from wikipedia:

In a business book by James C. Collins called Good to Great, Collins writes about a conversation he had with Stockdale regarding his coping strategy during his period in the Vietnamese POW camp.[18]

I never lost faith in the end of the story, I never doubted not only that I would get out, but also that I would prevail in the end and turn the experience into the defining event of my life, which, in retrospect, I would not trade.[19]

When Collins asked who didn't make it out of Vietnam, Stockdale replied:

Oh, that's easy, the optimists. Oh, they were the ones who said, 'We're going to be out by Christmas.' And Christmas would come, and Christmas would go. Then they'd say, 'We're going to be out by Easter.' And Easter would come, and Easter would go. And then Thanksgiving, and then it would be Christmas again. And they died of a broken heart.[19]

Stockdale then added:

This is a very important lesson. You must never confuse faith that you will prevail in the end—which you can never afford to lose—with the discipline to confront the most brutal facts of your current reality, whatever they might be.