yeah, i get what you mean.
Last year for Biology. i was literally gettting ABB or AAB. I even got a B for the experiment (WHICH WAS HARD), but when i got to the exam, literally got U, U
it killed me.
but i learnt something from it.
It was that when i get nervous in exams. I struggle and make mistakes. having no confidence in your work and yourself loses you mark. So get a good sleep the day before and make sure you have done all the past papers and are confident and comfortable with yourself.
Give yourself confidence and do it !