Self-Doubt is a mask worn over the mask of Self-Destruction that is worn, ultimately, by Fear. Self-doubt: I-don't-know-if-i'm-good-enough. It's a mask that says: maybe I should never have started. Self-doubt says: am i - is it - will it be ever be good enough? Do I actually suck and no one is telling me? Even when they congratulate me and pat me on the back? Even then? Will I ever make it? Am I ever good enough?
The Wright Brothers: for some reason or another they come to mind. Two guys trying to fly a plane - to lift this thing off the ground. I'm sure they heard more "You'll never make it" than "I believe in you." So they worked daily on believing in themselves. In the end, though, math doesn't lie and their math was, in the end, sound. So they succeeded.