if it's something you're self-studying, don't be afraid to put something down and pick it up later. There are topics and materials that I completely failed to understand the first time I encountered them, but months or years later they were easy to understand.

Of course, if your goal is to get good grades in specific courses which have exams on specific dates, then this isn't very useful advice.

Some other advice:

- don't underestimate the value of memorization when it comes to definitions and important theorems.

- do lots of practice problems which are like those on the exams (obviously)

- as much as possible, find peers to study with and talk with. it's easy to fool yourself into thinking you understand something, but when you try to talk about it with a real person, the holes in your knowledge are revealed pretty quickly.

- if you can't do that, try to explain the concepts and ideas you're learning to an imaginary friend.

- you might be able to find lecture notes or slides online from the actual versions of classes taught at your institution.

I agree you should ask the professors for materials to look at, or at least try to focus on the materials and courses actually taught at your institution. This is the path of least resistance and will probably make it easier to stay motivated too.