Me too! God, its the worst! You've practiced and practiced, and then you end up feeling like a moron. I find that working your way from performing with a large group down to doing solos helps. Baby steps. I've gotten to the point where a big group is just plain fun, quartets and duets are a bit of an adrenaline rush, but don't bother me as long as I know the music and trust the other people I'm working with. A solo where a larger group is backing you up quietly is less scary than a pure solo, solo. A solo where you are background music and people are chatting and drinking cocktails is less scary than one where everyone is sitting politely and giving you their undivided attention.
The more you perform, the easier it gets. If you only do a few concerts a year, it's no wonder you get stage fright. If you are performing in lots of different contexts every week, or several times a week, it will get better. If you really want to tackle this, try to get as many gigs as you can, formal or informal, with different groups, and without.
One thing that really helps for me is to practice when other people are around, instead of hiding away in secret until it sounds perfect. It's scary as all get out at first, but when you are comfortable sounding like a moron, not knowing the music, getting notes wrong, in front of other people, who don't actually want to hear you, suddenly, performing later becomes much easier.
I also like to tell everyone I see the day of the performance that I hate performing and that I'm about to throw up or pass out from fear. It makes me feel better to have said it. The scariest thing is being up there and knowing you sound bad because you are scared, and worrying someone else thinks you just are bad and haven't practiced. If I tell everyone I'm a nervous wreck ahead of time, I can feel all their good vibes coming back at me from the minute I get up there. They want you to do well, and they'll give you a subtle thumbs up from the back row if they know you need it.
And plan ahead for the nervousness. Know you will be nervous, accept it, and move on. Don't expect yourself to calm down and be perfect. That's just going to freak you out more. Oh my god! I have to calm down! Why can't I calm down? !!!!! PANIC!!!! Instead, get up there and go, yep, I'm nervous, but I'll be done in 5 minutes, and no one else out there in the audience could do it instead of me if I chickened out now, now, what do I need to actually be thinking about? The stretch for that chord coming up… and the tricky rhythm… Give yourself permission to be nervous, and plan to minimize the effects. Do you need to remember to play a little slower than you would when you practiced, because your hands are shaky? If you sit just right will that help keep your knees from shaking? Do you need to do a few jumping jacks or stretch all the way down to the floor just before you start to loosen up? One of the tenants of good classical singing is breath control and I can't breathe when I get nervous, so I just give up on that particular tenant entirely for solos. Oh well. Can't do that. I'll concentrate on a good sound instead… and I add in all sorts of extra breaths where I would never put them if I wasn't so freaked. I know I'll need them, so I mark them in the music ahead of time. I know my knees will shake, so I own some floor length skirts and knowing that no one can see them makes me feel better.
It helps too, when you've survived a few calamities fairly gracefully. The accompanist who gets two pages stuck together and disappears for a few bars, but you keep going and they catch up. The sneak attack: Jim is sick. Can you play this? Five minutes before you need to be there.
Oh, and record yourself practicing and play it back. It's hard to get a good sense of how you sound while you are in the moment, playing. You are concentrating on playing, not listening. Seeing and hearing a recording can give you some confidence. Things that you think are painfully obvious mistakes, turn out to be rather subtle, when the overall tone is good. Or you may notice that you sound fine, but your bad posture is really distracting, or that you look like you are miserable. Seeing yourself from the audience's point of view seems to make it less scary somehow.
You can do it! Whoo-Whoo!