I've been there as a performer.

I'm a vocalist for traditional folk music and let me tell you. My anxiety was so bad, I used to cry before my performances. I mean, really cry.

The best thing, I found for me was trying to calm myself through some sort of meditation. Just try to keep calm, if you're overwhelmed try going for a run somewhere or just a walk. Remind yourself that you know your music, and the people listening to you are there to listen to you, not kill you and you should be okay.