Your question is very common, especially for beginners of advanced mathematics. Study habits vary from person to person, mainly by yourself. To put it simply, we must first understand the basic concepts, especially the very important concepts such as limit, boundedness, continuity, differentiability, differentiability and integrability, and understand the relationship between them. On this basis, we should improve our computing ability, especially reading more examples and accumulating solutions.

When you say that you can't remember or understand theorem derivation, don't take your time. This is not to say that theorem proving is not important, but you should not focus on memory, but should understand it, so that you can remember the conditional conclusion of the theorem and understand the meaning and proof process of the theorem in the future. At first, the concept of limit is difficult to grasp accurately, especially when some problems are proved by definition. There is no good way here. Read more examples and imagine the process of limit in your mind. At this time, abstract thinking and logical reasoning ability are very important. This is the first difficulty and the most important knowledge point in higher mathematics learning, and it is much easier to surpass the past.