I used to struggle with how confident I should be. It requires a certain amount of confidence and naiveté to make it in business. But I've also met cocky founders who miss the things that'll kill them because they think they know everything.
Ben Horowitz offers a more interesting way to look at it - confidence is irrelevant; courage is all that matters. It is courage, the ability to do the right thing regardless of how easy or hard it is to do, or how you feel, that matters.
To steal from the post (worth a read): "Every time you make the hard, correct decision you become a bit more courageous and every time you make the easy, wrong decision you become a bit more cowardly."
Also, check out this interview (below) where Ben talks more about courage, the hard things in business, how tough Andy Grove is, and why there are still lots of social problems in tech.