hi, I was in the same place going into my a levels - anxiety is so horrible!
Have you spoken to anyone at school about it? It is possible you may be able to take exams in a separate room if that makes you feel more comfortable and also be able to take breaks if you feel panicked.
GCSEs can be really tough and don't listen to the people who get all 8 and 9s and when they say they haven't revised they are probably lying! Do your best and that is all anyone can ask.
Really hope it goes ok for you, if you need any advice or help do just let me know and I'll try my best to help