There’s a few things going on here:
Firstly your GCSEs need to come first - so make it like your job. Get used to studying and using a method that works for you. I would advise to revise as you go along- things like flash cards, mind maps etc., a little and often rather then cramming at the last minute.
Your after school clubs are great to relax and unwind. Do have them in balance with your school work.
Lastly, look after your mental health, so sports, quiet time, eat healthily. If at any point in time you begin to feel unwell with stress go see your GP as they will help.