It's not that you are stupid, but that you didn't study hard. In fact, any subject is not difficult as long as you study hard.

You need to do the following:

(1) Listen carefully in class and ask teachers and classmates if you don't understand the knowledge;

(2) Do more exercises after class and do exercises related to each knowledge point until the knowledge points are integrated. If you have questions that you can't answer, ask teachers and classmates until you understand them.