This link gives a taste of the kind of thing you can read in a book I highly recommend: The Talent Code: Greatness Isn’t Born, It’s Grown by Daniel Coyle. In order to learn a lot, you need to 

1. put in the hours and
2. study effectively during those hours.

To study effectively, you need to do a lot more than “going over things” several times. You need to identify what you don’t completely understand and zero in on it to figure it out.  You need to wonder how each idea relates to everything else you have learned. You need to get to the point where you can write your own exam questions and answer them, carefully thinking through what it would make sense for the instructors to test.    

The human mind has a natural tendency to skitter away from things that are hard to understand. Effective study requires you to resist that tendency like a bad addiction. You need to turn toward the ideas that are hardest, not away from them.