I have come into contact with many students, who always claim to have taught themselves many mathematical theories, but they can't even do basic calculations. So I generally suggest doing more questions.
Here we would like to add two points. First of all, if you teach yourself high school mathematics, there is no need to waste too much time on the sea. They won't make you better. Secondly, you can do competition questions to improve yourself (middle school stage), but don't invest too much energy.
2 Develop the habit of taking notes. While learning new things by yourself, rearrange and summarize according to your own understanding. Many times, when you teach yourself math, you will skip a lot of things. When you think you understand, you don't really understand. Taking notes can make you slow down and appreciate math carefully.
For example, if you look at a very long certificate. You think you understand every step, but in fact you probably don't really understand why he did this step, that step, why he took the time to deal with the details of the step, and so on. When you carefully organize this proof according to your own understanding, you will deeply understand the author's intention.
In recent years, I learned a habit from one of my younger brothers, that is, while learning new things, I organized them into handouts with TEX. Although it takes some time to do this, it is very beneficial for me to understand mathematics.
Understand mathematics instead of reciting it. Among the students I contact, too many are reciting math.
A professor once told a joke about his experience. He said that one of his classmates is a schoolmate, and he has read far more math books than he has. Once, he went to ask Xueba what a complex and changing concept meant. As a result, the man just recited the concept and could not explain it in common language with his own understanding. Therefore, in fact, this schoolmaster doesn't really understand this concept. Such a person can't learn math well. I collectively call them "Masu-style" self-learners. There are too many people of Masu type. Last semester, a graduate student claimed to have read all the advanced theoretical books, and as a result, he couldn't even write a simple master's thesis.
Anyway, how to understand mathematics instead of memorizing it? For students who have not been trained in scientific research, the most practical methods are the two strategies mentioned above. In addition, when reading math books, we should try our best to understand the author's intentions and grasp his ideas, especially when reading proofs.