Chill baby chill! (2)

One good way to calm your nerves a few days before a performance is to play your music slower. Playing at a slower tempo allows you to relax and gives you a sense of easiness which releases the tension in your body. Once you feel more relax, you can then gradually speed up up back to the desired tempo and you'll feel much better. 
You can do it every day if you need. Once you can already play your piece fast a few days before the performance, what you need at this point is to get your mind and body in the best possible shape to perform at your best.