This happens to me sometimes. I think it's because we have an idealized vision of ourselves, and whenever we do something that conflicts with that image, it calls everything into question: "If I am not that image of perfection, I must be something else... something lame and bad." When other people see our non-ideal self, you've got an additional layer of embarrassment to go with it along with corroboration for the conclusion that we're not ideal. That's my theory, at least.
You can overcome this by accepting yourself for the imperfect being you are.
...good luck with that.
NF's are the perfectionists of the world as I understand it. I agree with what's being said here. It's the dark side of demanding perfection... that nothing quite measures up, including yourself. Therefore I think NF's in general probably tend to be hard on themselves, though I think that it's more likely to be found in INF's who are naturally more reflective.
This strikes a cord for me personally just because I have been an awful critic of myself in the past - totally, unrealistically intollerant of shortcomings. It's something that I have tried to work on.