Firstly, you are not defined by your grades, you don't need all 9's to get through life (unless you're aiming to go into something like medicine) no matter how many people pressure you to get them. A-Levels are the big ones that universities look at (whether you're thinking of going to uni or not, i'm not sure), and they normally just want at least C's (sorry, I don't know the equivalents in England) in core GCSE subjects and maybe a few more subject specific ones.
I was similar where I just could not relax when it came to GCSE's, it would make me anxious to revise and think about it but whenever I'd put it off and do something else, I'd feel as though I was procrastinating and going to fail. I'd say maybe have a little look through your textbooks or notes, and do a bit of highlighting of key things you are going to need to remember (i'd say this is more for sciences, maybe some maths equations you need to remember, if you do languages, this may be good for phrases/words to remember. just pick a subject, preferably one you enjoy the most.). Don't think about it too much, just have a little bit of fun with highlighting through. These will help when you go back through your notes to find the information better with less effort than you would originally have to do. That's all for that moment and see how you feel. Whenever you may feel like it, (earlier will help you out overall though) you could make them into questions onto flashcards. While you do it, you could put in some headphones and listen to music or put on a TV show/movie in the background to have something else you can focus on if you find yourself getting overwhelmed or panicked. When you have flashcards written out, you'll have them at the ready when exam season comes around and you can just whip them out when you need them rather than spending your time rushing through writing them just a couple weeks before the exams. (Oopsie, I was guilty of this) If you find it super difficult to find a place to do some revision without being bothered, then maybe (and I know it sounds odd) you could say you're 'having a shower/bath' and spend even just 10 minutes having a little time to watch youtube videos to understand topics, to write out a couple of flashcards or anything like that.
If you find yourself panicking, have a little break and settle. Mental health always comes first and don't beat yourself up about it. Everyone finds themselves struggling around GCSE's, a lot of people struggle with the same worries and psych themselves out with not being/doing good enough. Try not to let GCSE's play on your mind too much, I know it's easier said than done, GCSE's can be brutal and don't deserve so many people stressing and overthinking it 24/7. A youtuber who I wish I had seen before I finished my GCSE's is Unjaded Jade. She goes through effective studying techniques and also just radiates positivity.
I hope any of this helps in any way, and I promise you'll be okay and get through to the other side of it.