Learning and remembering

I’m not entirely sure what it is. But I find myself on a topic will learn it and do fine. Go back to it and completely forget how to do it.
I’ve never been great at exams. But struggling to retain the knowledge or calculations.
For example Will do IHT, go away from it for a couple of days and completely forget the calcs/computations. I can not figure a way to grasp the techniques and retain the knowledge.
I come to the mocks and go blank. Bearing in mind I have been studying this since July.

Any advice to help me would be great.