First, you can read the textbooks yourself. General textbooks are more basic. First, you understand the content of the textbook, and then try to do the following exercises.

Second, after clarifying the content of the textbook, try the content of other information books.

Step by step like this. For you with poor foundation, you must start from the foundation, one step at a time. Although you can't keep up with others now, if you constantly consolidate your foundation every day, you will slowly fall behind.

You need to stick to it, otherwise it's useless. Also, ask others for advice if you don't understand. Everyone has something you don't understand. No matter whether the question you ask is simple or not, you should have the cheek to ask. Only in this way can you make progress. In fact, math also needs you to remember something. Of course, you can choose to understand memory or learn by rote. Of course, the former works best. There is also the need to practice more questions (starting from the simple ones), because only when you read more can your mind be opened, and only when you read other questions can you recall that some of the questions you have read before can solve the current problem.

There is also a very important factor, you should cultivate your interest in mathematics, which is of course linked to your own mathematics level. The first thing you should do is not to hate it, otherwise you can't learn it well. You start from the simple, as long as you can be simple and slowly complex (complexity is just the synthesis of many simple problems), your confidence will rise.

I hope it helps you.

You can ask me any questions.