Mathematics is not something a normal person can just read once and then use. You need practice and experience. You can read all the theorems you want, study all the big books and works, and still, you will have forgotten most of it or not gotten the idea.
What helps me the most to understand something, is to translate it into my own language. I have a certain amount of knowledge and a certain way how to see things. You might notice that the very same thing can be seen and discussed in various different ways, e.g. linear algebra could be done in matrix notation or in map/function notation. The first step to understand a new work is to find out how it embeds into what you already know, how it translate into the language you are using.
Another important point are repetitions. You can't just read a mathematical text once and suddenly you know everything in there. But, different from other topics, reading it 100x also doesn't help, learning it by heart you will never fully understand it. Mathematics is very much learning by doing, even though it seems rather theoretical at first sight. Thus, if you really want to understand a topic, use it yourself. There are multiple ways to do it. For example, the authors might sometimes leave out proofs as they are easy or they just say "this can be seen by applying theorem X.Y to the special case of ...". Do not just read over such passages but take them as an exercise: If the author of the book thinks that you, as the reader, should be able to prove what he wrote there, then go on, do it. This not only helps you develop a deeper understanding of the topic and the tools to use it, but also repeats the previous theorems, as you will most likely use them. A textbook that comes with exercises (and maybe even solutions for them) is of great help here when first learning a topic. For example, you might consider looking for higher grade textbooks at your university and try to get your hands on the exercises and solutions from a few years back.
As a third point to help understand and memorize a topic, try (after doing the first two points) to filter out the relevant parts. Assume that you are preparing for an exam on this topic and you are only allowed, say, one page of notes to bring, so you should choose them wisely. After you did the proofs and the exercises in the book, you should be able to see which results are important to use and which ones are technical and are only there to help prove the main theorem - thus will most likely not be important outside of this chapter.
I know I did not answer your question on how to learn more in a setting, but that is because in my opinion, you can't. There is a certain amount you are able to learn, for some it is more, for others it is less, but you can't just go and fully understand the contend of a whole semester on a weekend (you could read through it maybe, but you would still not be able to use it). What I tried to give you are hints on how to properly study on your own, how to make sure that it does not go to waste and that you really memorize what you did, that you will be able to use it even outside of the setting of the book/text.
For me, my limit when I really want to learn (as described above) a completely new topic from a high density book is maybe 10-15 pages a day. So 4x12 already sounds quite much, that is the reason why I assume that you are not properly learning it and hence will not be able to use it fully.
However, don't loose your motivation: If you extend your knowledge and remember the first step to translate it into your own language, then you will notice (rarely at first, more often as time goes on) that a topic is not completely new but rather an extension of what you already know; maybe a different point of view on a topic you already studied. This allows you to go through it faster (as you might for example already know some of the tricks used) and on the other hand also allows for a fourth point, the one that is the most fun of them all:
Apply the new theory to extend and simplify your knowledge. It is often the case in mathematics, that a fact that was really difficult at first and took the professor hours to prove, becomes easy and obvious once you have developed the right language to properly understand it (often only years later). Revisiting the points that caused you problems, maybe even open questions or little riddles you found for yourself, later on with new knowledge and seeing how some of them just resolve like nothing is simply beautiful.
edit: One of the comments above reminded me of a fact I found out through teaching as a student assistant: I have only fully understood a topic, if I am able to teach it understandable to others and motivate it for them, show them how what they learned in the lecture the day before and that sounded really strange is in fact not that hard but rather nice. If you get the change, go for a part time job as a teaching assistant, it will really help you to review topics you once thought you fully understood and see that there are even more to learn there.