I have the same problem. I think you just gotta go for it. That's what I'm trying. I'm just trying to be comfortable with the situation, and imagine like I'm alone and nobodys watching. Just like the saying: "Dance like nobody's watching." I think people made this up becuase they realized some really great people out there don't show their talents and they need to open up to go far! That's how I feel anyway. Good Luck! Hope I helped.
Practice A LOT.
Every chance you get, perform for someone before your big performance. Whether it be your family, friends, someone you just met. And ask for feedback.
When you're on stage imagine yourself in the audience's shoes. Imagine you're someone else watching you. What would you think? Would you even be thinking about judging this girl's performance? That works a lot for me... because I used to think people judged me when I sang. But what goes through you're mind when you watch someone? Usually a whole lot of nothing right?