I came across Dick Costolo's post....with this quote:
On a psychological level, I think a lot of people confuse fear of failure with not having enough confidence in the ultimate success of their idea. They thus conclude that they aren't confident enough in their idea or their strategy because it seems to have holes and flaws for which they don't have answers. This is a tremendous mistake. While I won't pretend to speak for the entrepreneurs I mentioned above, I bet if you asked them if they were confident on day 1 that they had a winner with each of their previous successes, they would look at you sideways and say "of course not". Speaking for myself, I can say that my cofounders and I try to find a market opportunity that seems like it will need to be addressed and for which we think we have some angle and then we just pull out shovels and start digging and figure other things out as we go.
The entire post is worth reading.