What I've learned is that you can keep going long after you think you passed what you believe is your breaking point. When people say they "don't have it in them?" or they "can't go on", is in every sense of the word a cop out. The power and drive inside you can sustain any failure or weakness of the body. Once your mind is set, there is very little that can stop you. Moral fortitude and tenacity will carry you through the darkest of shadows.
It is so easy today to give up. We warn kids participating in sports program that losing is expected and to know that it doesn't matter. What???? Are you serious? In the adult, real world, losing or it's matured equivalent is NOT to be expected or accepted. You can't take your boss a half-assed proposal or business contract and expect him to understand that you were having an off day. It doesn't work like that. We have stopped trying to attain excellence in every endeavor and we teach kids that is to be expected. What??
Self-fulfillment and achievements are feelings that cannot be measured and therefore cannot be imagined until felt. That feeling once you achieve it is such a rush and feels beyond measure. That need for achievement should drive you through your lifetime. It is healthy that you try to one up yourself. It is the only way we can change the world.