I was pretty much in the same boat as you when it came to my GCSE's, when I took the mocks I did work myself up a bit but in the end, they were mocks, and I never 'panicked'. However the moment study leave came around I just crumbled, I was found crouching in corridors more times than I care to admit and I literally had no idea how to control myself.
Luckily for me, I had a friend in the sixth form who, on one of my very dark days, bunked off college, took me out for a cup of tea and let me know that I really wasn't alone and that I had to start making changes for myself that worked for me, and forget about what the school wanted.
We got back to school, marched to my year office and asked for seating near the door (so I could escape with as little fuss as possible if I needed to) and requested that I wasn't forced to attend 'turbo sessions' -these were compulsory revision sessions put on by the school during study leave that just sent my anxiety through the roof.
Getting into the exams was never ever easy, but it's in the schools interest to get you into that exam with the highest potential to succeed, so if you go to them and tell them how you're feeling and suggest the ways in which you think they could help you - it will be far more beneficial to you than them giving you the bog-standard 'yeah they're freaking out we'll just sit them quietly in a corner' treatment.