Why am I getting bad grades even if I study hard? Need suggestions.

I have seen students work during the semester and still got good grades. While I addicted all my time in learning the school materials and still got 50-60%. I find it hard for me to follow the teacher in some classes. And I try to study on my own. the textbook took me forever to read through it, and when I started doing the problems, I have no idea where to start . And also because the reading takes me too much time , I have limited time doing problems at the end of each chapter I also tried not to spend too much time textbook , like using the teachers notes or slide plus some reading . But I always feel that I do not understand the materials well. I find that very slow learner, which means I literally have no time do other things like my friends do. I would appreciate it if someone can give me some advice on this , thank you so much 😊