How do you remember what you have learned in the past?

I'm currently enrolled in a half year math course, where we go through one chapter a week from the textbook, which is a requirement for the school i wish to attend, and it has been going well, or so i thought.

I decided to go back in my notes to revise what we've been through thinking this wouldn't be a problem. But when i started trying to solve problems i couldn't remember how to do them, even though at the time i learned and understood the material and could solve equations without much hassle, and this is really annoying me and I am stressed about the exam.

My question is: how do you remember how to solve different equations in different types of math after some time has passed?

Thank you for any feedback and sorry if this is a stupid question.