it sounds like total waffle but trust me: reframing.
it's okay that you're struggling and yes we'd all like to do better but sometimes you might stress yourself out so much trying to do better that you actually limit yourself. you're struggling with the sciences? no biggie! we've got more time than we think but we also waste more time than we think on stressing about grades.
-do your best
-stick stuff to your walls and recite it while getting ready (for things like formulae or systems)
-do practice papers - take note of where you make mistakes and go over them (make notes, flashcards, whatever works best for you) and also look for weird questions that keep popping up, because sometimes the examiners phrase really weird questions
-don't be afraid to ask for help from teachers/peers/family/forum users
good luck and hope this helps x