I am a mathematician.

I would say the absolute best thing you can do is take the problems you DO know how to solve and write out the solutions, using complete sentences. If you can't explain some part, then you will quickly discover which parts you are following by rote.

At that point you need to go back and understand the process. It will be a process of writing and writing, revising, and understanding. It's not an instantaneous process and may take a few months to a year, but it is a rewarding process.

You may find you will have to think about certain concepts you thought you knew, even basic ones. Don't worry about that. It took centuries for mathematicians to even write down precisely what a set is for example.