I'm also doing my GCSEs at the minute, and coping with the stress by making sure I feel like I have done the work. This involved lots of printing off excel coloured in timetables, and copious amounts of blu-tac but I have a wall of revision and work. Im pretty stressed about it because the only allowance I could be offered is extra time, according to my teachers, which isn't particulary helpful as I don't need it, I need a room that's not full of scratching pens and paper rustling constantly while I'm taking the exams. I've always under performed in big exams compared to smaller tests, but I've definitely improved.
I suggest you take a deep breath and think, you're probably going to end up doing some other qualification/course of some sort after GCSEs, so you might as well try and embrace the stress as your new ordinary life. This might help you accept the stress, and stop it getting in your head.
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Female, 16
Knowledge is knowing that tomatoes are fruits. It takes wisdom to know not to put them in a fruit salad.