I've been paying the bills with my startups for over 7 years, and I still drown in self doubt.
A technique I've used to short circuit that thinking is simply this: Ask yourself every day, "How can I accomplish this?"
For some reason, forcing myself to ask the question How, crowds out the doubt. My mind starts cranking on solving problems rather than worrying about my ineptitude.
I'm always impressed by how well just putting myself into a problem solving mood works.
It was awesome to see Daniel Pink's most recent book, To Sell is Human, explore this. He showed studies of how a group of people who were put into a "self-questioning" mindset solved 50% more puzzles than a rather than folks in a "self-affirming" mindset.