I was struck a few years ago by an interview with an olympic athlete. She was asked how she found the discipline to keep a grueling practice schedule. She answered that what many see as discipline is actually passion.
If you're having trouble finding discipline, the answer may lie in finding a way to love what you're doing more.
This is very important. Having the iron discipline to work very hard for a long time at something you don't really like, in order to achieve some unrelated goal (for example getting rich) is also known as obsessive-compulsive disorder. The worst thing is that this approach sometimes works - I have met guitar virtuouses who definitely have this disorder. But if it's at the expense of your well-being, it is probably not worth it.
There is a lot of this in the startup community. I'm pretty sure the early life of Cisco is a pretty good example, but unsuccessful instances are all around you.
