I felt like that when I was studying my alevels. Felt like everything I was learning, wasn't sticking to my brain.
It's probably useful to constantly test yourself and do past papers. Try to relax- don't get yourself stressed out.
For some odd reason, when I sat my exams, everything just came back to me beautifully! Like I remember some of the content even now so clearly haha!
Just keep working, don't let the stress get to you, relax, meditate, listen to calming music and have a hot drink! You don't suck! Don't think so negatively, you will do amazing, keep telling yourself that and it will happen