Don't count yourself out. It sounds so trite, but do you realize how often you already do it? Not just in start-ups, but everything: Do you not talk to that girl/boy because you don't want to bother them, or think they'll say no? If you're not convinced you're worthwhile, how will you convince anyone else? Are you so concerned with being right that you won't let yourself become absolutely certain of your own success because "there are no absolutes"? Are you worried that if someone takes a chance on you, you'll let them down? Does that worry stop you?

Most people stop even trying and take themselves out of contention!

Rejections are like points. There's a secret magic number you need to collect, and then you succeed.