I'm in my 3rd year of pure math bachelors, and a thorny issue keeps on reappearing for me. I understand the theory, for a day, maybe two. I've reached the point where our lecturers no longer detail every single step, but leave some details to be filled out myself. Most of the time I have no issue filling in the blanks myself, I understand everything, I can follow the line of thought. But after a few days I just forget everything, conditions for a theorem, etc. Even what I studied yesterday can be forgotten today. I feel like there is a finite box inside my head, and after its full if I want to put something in I have to push something out (involuntarily).
Are there ways to solve my problem? It's very worrying for me I can debate with my friends about the theory when I have access to my notes, but keep on failing theoretical exams (written or oral).