I've come to the humbling realization that I have a breaking point.  It's all in my head, but it consists of a combination of physical, emotional, and psychological components.  When one of those components is missing, I'm strong and confident, capable of anything and everything.  But the right combination of those three things breaks me.  I give up.  I quit.  I make irrational decisions and have a hard time coping.  When I think about it later, I feel stupid and weak for not pushing through it.  But that's because I'm not in the situation anymore.  It's easy to feel unbreakable when I'm outside the situation that broke me.