...I've noticed this phenomena in people before, it very often goes that the better you are, the more insecure you are, despite the fact your performance sounds wonderful to everyone else, to you it sounds like shite, you know where you fluffed that note, you notice where you sang slightly off key and because you are good at what you do you know you could've done it better! But remember, most of the audience don't know where you played the bum note/sang off key or whatever cos they were sat there just enjoying the whole performance and very possibly are nowhere near as picky as you.
SO DON'T GET DEPRESSED ABOUT IT - CEASE THE NAVAL GAZING AND YOU MIGHT ENJOY IT MORE!