Concentrate on breathing in through the nose and out through the mouth, it should relax you to focus on the most important thing, the instrument. It's different to acting where you might have to play to an audience, as in develop a rapport, because as a classical musician you only have to sound good, but damn good. Only focus on the positives, like: "I know this song so well", "I've played it brilliantly many times" and so on, concentrating on the music and the music alone. If you think positive and focus, it will fly by. If you go out there all the time thinking, "Oh I remember last time, when I shook..." it will happen again and again. Be positive and trust your ability, and take heart that you're one of a select bunch who can perform the skill so well that people want to hear it :)