Revising so much and still getting Us

I'm in year 12 rn and I do bio chem and psych at a level and my target grades are BBB but in bio and chem I'm literally averaging Ds. I revise so much and i know its mostly about the quality of the revision more than the quantity but no matter what i do i just can't seem to get the top grades, what am i doing wrong? I watch videos, i do past paper questions, i analyse my end of topic tests to see what i can improve on but nothing works. Any tips?? I'm starting to think I'm just a bit dumb lol