Self-perception is key! Wow... and you see this all throughout human interaction! You can only be as good as you see yourself to be!
If you have low self-esteem, or you feel lacking in some respects, whether or not you can actually do something because irrelevant, you started yourself out with a handicap.
You have to SEE yourself as such! If you can't, if you don't want to see yourself that way or if you don't believe in yourself: You won't!
I would publicly confess, I've tried to teach that to a couple of people I've known... But, I guess it can be very hard sometimes.... Living is such a fun challenge.