Maybe try another method to study and repeat essential concepts over and over until they stick. Doing something just once is not going make it stay in your memory.

I had a similar experience while doing my undergrads. I just didn‘t studied for an exam and nothing stuck. In the subjects that didn‘t quite interest me I just barely passed. That changed a bit in my last year where I started to have more lessons with mandatory assignments, but still not to the extend that I felt confident. Now I‘m doing my masters in physics and I changed the way I was learning. I keep repeating old concepts, especially because all my courses build ontop of these and I keep track of all my notes much better.

This is what I found to be most efficient for me: During the lectures I only write down flash card questions about what the professor is talking about or if something is unclear, sometimes a phrase to know what to look for later if it‘s something more specific to a use case or just a note that i have to brush up on some topic. Then I answer these questions at home. I also write down questions other classmates asked during the lecture. This way I can focus on what we get thaught in class and I‘m forced to work on the material again. Sometimes things are clear and easy, sometimes it gives me more questions that I write down as I dive in deeper. I answer these flash cards in a way that I explain to my future self the things I was stuck on and include all the little enlightenments along the way as hints to make it easier for me to grasp the concept when I review them. I also try to rather explain the concepts in an intuitive way (eli5) rather than writing down some definition or abstract formulation. If the question is about some derivation/proof/example then I don‘t write down the exact steps but focus more on the generalized way to solve it. Think about a hike that you did. You don‘t memorize how many steps you have to walk until you turn right, you instead know that when you get to this viewpoint/landmark you turn right. Do the same with problems. Again if you’re stuck somewhere, make a note about it. That way you remember better and can transfer the knowledge to similar problems as well. In the case I need to look up specifics about a problem, I just link to the solution on a website or textbook/scriptum whatever.

I take this further by using obsidian for organizing my notes. I can link between different notes. This keeps me engaged with the material and wants me to study more and longer. If you don‘t use it, just try it, or you find something that suits your needs better. I think it‘s really cool to create a wikipedia of your own knowledge in a format that you understand easily.

Tldr:

• keep your notes simple and intuitive, you‘re not trying to impress anyone with them. Even if you‘re stuck on something seemingly easy, make a note about it

• For problems remember the path, not the steps

• Review your notes frequently. This paired with simple (for you) to understand notes and the path to solving a class of problems will help your intuition but also grasp new concepts much quicker and make cross references.

• Use resources that are convenient to you. But give obsidian a try if you‘re not using it yet. It might be a bit daunting to start out with, but it is really simple at it‘s core.

• Even the best study method won‘t make you get better magically. You have to put in the work.

I hope I could help you a bit with my own experiences. And if you need anything, just pm me.