I'm also not very smart and it generally takes me longer to learn things compared to my peers. I also have a tendency to make a lot of the same mistakes over and over again and to get things mixed up a lot. It makes university a pain in the ass to deal with. It's also why I've never been that good at math, even though I honestly admire the subject and wish that I was good at it.
Have you ever tried looking into peer tutoring and going to office hours? That might help you out a bit. If your lectures are recorded then you could try watching them online and taking notes. I find that it helps me when it comes to trying to understand the content. You could also try going over the textbook and also googling some of the concepts. I find that this helps a lot too. It's also good to try and reread your notes as well. The issue also might just be the type of courses you are taking, so you could try taking other courses to see if there are some that might click with you better.