My only advice is around revision. Best thing I found is revision cards and practice questions. You can only do your best. I understand the pressures and it sucks.
The other thing I’ll say is personally, my grades were very average (mostly C’s and 2 B’s). I went on to A Levels and undergrad and masters. Now I’m on a PhD. GCSEs aren’t the be all and end all they’re made out to be. My partner got 3 GCSEs, failed his A Levels (due to health problems) and went into a foundation degree and got an art degree. He sells his art for a living. I know my answer is very uni based and you might not want that but GCSEs aren’t *that* important down the line.
If you wanna improve your grades though, practice papers are probably best. Don’t put too much pressure on yourself. If you feel yourself burning out, stop and do something you enjoy then go back. Good luck. I hope you feel better soon