Day...16? Question

I believe that if you want something bad enough than you go get it, or you do it. I have always believed that. The problem is .. where does the motivation come from. When you're in a dark, deep hole and the walls are closing in, and you know what you NEED to do... how to you push yourself to do it?

I know this question sounds like I'm in a bad place right now but I'm not, and that's why I can ask this question. I just simply want to understand how some people can keep fighting even when the world has knocked them down repeatedly and some people just stop at the first sign of trouble.