From Steven Pressfield excellent little book The War of Art.

Self-doubt can be an ally.  This is because it serves as an indicator of aspiration.  It reflects love, love of something we dream of doing, and desire, desire to do it.  If you find yourself asking yourself (and your friends), "Am I really a writer? Am I really an artist?  Am I really a _________?" chances are you are.

The counterfeit innovator is wildly self-confident.  The real one is scared to death.