I saw a good quote recently which sadly I can't find now to repeat verbatim. It read along the lines of: 'Self doubt is essential. If you swan around your studio convinced you are a genius you probably going to produce crap.'
The author of the quote just got in touch - here it is (thanks, David): "Self doubt is important, because if you're knocking around the studio like the Fonz, you're probably gonna turn out a right crock of shit." David Dipré