Some motivations last a lifetime, I don't think our desire to feed ourselves is going away anytime soon..
A lot of current thinking on the subject is based on Abraham Maslow's 1947 paper "A Theory of Human Motivation". Your "hard discipline" seems based on the "esteem" part of his "hierarchy of needs" while this might be a sufficient motivator for some, for many it will likely be beneficial to look for other motivators.
Honestly, xwdv is more right than wrong. If you lose motivation to work on your startup, your startup didn't really fail so much as you're doing the wrong thing for you.