Well it's not that hard... Normally smart people don't realize that they are smart because they have higher standards and often if they reached those they set them even higher. So from their perspective they normaly never reach their own standards. The problem is that smartness and self-confidence often seem to be diametrically opposed.
The point is: Don't hesitate too much and learn to deal with your doubts (especially if you start a startup :o)).
It is true -- sometimes I get that feeling that I beat my up unnecessarily for no real reason, and get miserable. Then, it takes someone else (like my mentor), to point to me all the things I did right, which makes me go "oh yeah ..", I'm not a that dumb after all. HEH.
But you know what, if there's one thing I've learned from my previous job -- working in a team where I am the smartest person on board, IS NOT APPEALING to me. I specifically told my current boss, during the interview, that I _don't_ want to be the smartest guy on the team.
So basically its an infinite loop:
1. don't want to be the smartest person on the team
2. play catch up
3. go to (1)
So in all that three steps, the standard I set for myself is always higher and I never appear to have "met" my standards. Just like you said.
The point is: Don't hesitate too much and learn to deal with your doubts (especially if you start a startup :o)).