If it helps any:
Confidence is generally projected. You can be nervous as hell but if you go out with your head high and your shoulders out people will *think* you're confident and things will flow better.
That sounds like overly simple advice, but it worked for me. If your brain works the same as mine, there's probably nothing you can do to stop the I'm-not-good-enough thoughts from butting in, and that's fine, let them come, just don't let yourself be paralyzed by them. If you act confident, most people can't the difference between real confidence and faked, and it's a lot easier to be confident when people treat you as if you actually were (if that makes sense). With enough practice, maybe it won't be fake, any more. I'm not quite at that point, but maybe eventually...
Both of these are excellent. I had to do a group project this year in which each group member had to do a presentation. I hate
doing presentations and frankly think the person who invented PowerPoint should've been strangled at birth. But I ended up with a group of peer reviews and several verbal comments that talked about how "confident" I looked while doing my presentation!
Behave "as if." "As if" you know you will do fine and "as if" you have few or no fears. Others can't see into your mind; they will see the confident person, and eventually you will find the "as if" confidence turn into the real thing.
Does that mean you'll never have doubts again? Definitely not. But I find that it gives me a better sense of perspective about what I can do and what I actually deserve, compared to what my brain is telling me at times.