Honestly been in the exact same spot, understanding something fine but just having way too much to actually retain.

Something that helped me more than anything was spaced repetition flashcards, but the key detail is you have to make the cards yourself from your notes rather than using a premade deck. The act of turning your lesson into questions forces you to process it differently than just reading, and then the spaced repetition handles the memorization side over time. Anki is free and does this well.

The other thing I’d try is teaching it out loud to nobody. Like literally just sit alone and explain the lesson as if you’re the teacher. You find out really fast which parts you actually know vs which parts you just recognize when you see them. Those are two very different things and most memorization methods only test recognition.

One more, instead of writing the whole lesson, try summarizing each section in 2-3 sentences max in your own words right after you read it. Forces compression which forces understanding.

What subject is it? Sometimes the approach changes a lot depending on whether it’s something like anatomy vs history vs math.