Performing in the west end in a month, How can stop myself from getting nervous?

Im 14 and part of this drama group that is performing at her Majesty's theater In just over a month. Were doing three numbers and one of them is a disney princess one, where we sing solos the disney princess' sing in the films. I've been picked as sleeping beauty and the song is once upon a dream, which is pretty high for me considering I normally sing low. I love performing so much, and had always dreamed of being in the west end when I grow up. But since I started secondry school, I have always been so nervous when I comes to performing. I will perform, because obviously, I love being on the stage, but I get so nervous before hand. And I dont mean just usually being nervous. I get so nervous that, in auditions, I cant speak, I go bright red, my hands sweat and Im shaking all over, I hypoventolate. Its horrible! and thats only in an audition in front of a few people! I never used to be like this before, so I was wondering why this has suddenly happened. I get most nervous when I sing. The biggest audience i've performed for is about 100 but this west end theatre is going to be twenty times that! Latley i've been watching britans got talent and seeing how huge the stages are makes me panic and I start to sweat and have to leave the room. Its horrible! I've tried everything to stop getting nervous, so im pretty much begging you to help me hear! I would really appreciate it! maybe theres some tablets I can take or something? It would really help! Thank you :) xx