I feel vindicated!
This is something of a personally important issue for me, I guess. I wasn't exactly brought up under the slogan of 'believe in yourself and anything will be possible', but it's a common enough refrain that I was familiar with the idea and sorely confused by it. I'm pretty sure I was born a realist, and my nascent love of science was very clear that if you thought you could do anything just by mustering sufficient determination the laws of physics were going to have some harsh words with you. (Everyone thought this was nit-picking, of course. I was also born a pedant.)
As I continued to try to justify my unfounded thoughts on the matter it also occurred to me that determination was far less important to your success than accurate judgement - since, after all, if your ideas are wrong your inner drive is worthless and if your ideas are right then your determination just hastens what is ultimately inevitable! At some point I discovered and became obsessed with the reductio ad absurdum and the idea that submitting your own ideas to reckless assault for the slightest weakness was the only sure way to guarantee you were right, to prove that it was impossible to be wrong! Oh, and G. K. Chesterton's introduction to Orthodoxy - "I can show you the homes of the supermen... the men who truly believe in themselves are all in lunatic asylums" - didn't hurt either.
So basically, I'm really enthusiastic about self-doubt, and I hope you'll forgive me if I point you in the direction of two much-better-written articles - at Scientific American and Nowsourcing - and simply seize a shamelessly egotistic opportunity to relish my own correctness.