I dont perform musically but I compete at dressage, and I know the feeling, I have actually thrown up with fear before I go into the arena.
I've got over this by smiling, smiling at everybody, the judges, audience other competitors everybody who I can see. I know thats not easy to do when you singing! but it helped me.

A friend of my brothers who does perform visualises the front row of the audience as being mates sitting in his front room at home. That worked for him, not instantly, but after a few months he cracked it.

Good luck.