I didn’t study any sciences or maths - sociology, gov and pol, psych, and I got BCD mocks but got ABB, My revision saved me. I would just memorise things. Learn a part and call it back to myself whether it be like i am teaching myself out loud or writing up essay plans. I’m not sure how different it is for stem subjects but exams are just a memory game. Make the content stick in your head by constantly recalling it - research method of loci