I'm a nervous person, always kind of hyperactive, but in terms of thinking, I even have some problems concentrating if I have too many things going in my head. That translates to the stage, and in addition to the classic stage fright, the result has been more or less a huge mountain of cr#p.
However, something changed this last trimester, when I had winter auditions at my conservatory. I didn't care a lot really, partly because I knew I was going to do a decent "show". And everything turned out great! Then came my classical audition, and even when the vibe was tenser (just you and your instrument in a room full of people) It turned out the best performance of the evening, mainly because it was the only one without any audible mistake.
I think there are two factors to observe here: the first is, as you said, how many times have you been exposed to a performance situation. It doesn't even have to be a gig, just performing in front of people that you respect or even are afraid of (teachers, your boss, your mother in law...). The second is that you really have to have confidence in your skills. That doesn't mean being cocky and going around the place acting like Miles, but being able to be at a rehearsal and actually believing "this is easy, I'll come out as a piece of cake on the gig". You have to be motivated, and you have to avoid bashing yourself, as many musicians do. Don't take the music that seriously, that way you'll play better ;)