It is difficult to learn advanced mathematics at school. If you have no foundation, I think you should preview more and review more. To understand the formula after class, you need to do more exercises to consolidate the knowledge points you have learned.
To put it bluntly, I think the most important thing is understanding. The effect of memorizing without understanding is definitely not good, and it is not suitable for long-term memory, and it will be particularly tiring to learn. Especially about the basic definitions of derivative, integral and series, it is impossible to learn these things well without thorough understanding.
Then we should not only understand, but also understand is the beginning of our study. After understanding, we should be able to remember this formula well, so the second thing is that our memory of the basic formula should be full marks. There are some basic formulas such as derivative, integral, series, limit and transformation of trigonometric functions. If you can't remember them well, there is no way to solve the problem. Then even if you understand it well and thoroughly, your advanced mathematics study is still a failure.
Finally, I think we should keep reviewing. The best review is to do more questions and practice more. Using the sea tactics, many problems have been solved, and it is natural to understand and summarize the solutions of each kind of problems, and similar problems will be solved soon in the future. Because our brain can only be in a short-term memory state when we start learning, we need to practice and summarize constantly if we want to remember for a long time.
If it really doesn't work, you'll have to work extra hard. It is suggested to find a teacher to make up lessons and study by yourself.
In short, if you want to learn something well, whether it's high math or something else, you have to work hard. Study in a planned, step-by-step and targeted manner.