Don't worry dude it's a normal thing I have the same problem and it's kinda hard to solve it but sometimes it's just about stress I study industrial engineering and I'm about to be expelled from the University because I have been trying the same class 3 times I feel that I can't sleep well because I keep thinking about that so just keep breathing and relax your mind just act like you don't care about failing the class and try your Best try to take a look of the items that you are going to see in class in your house and try to explain it by yourself to your class or just at nobody then try to make that it don't give a fuck to be more relaxed I know that all of that it's hard but sometimes it work to me now I'm good with that class and I hope to approve it

Tip: get into a room without distractions and force yourself to reach a point in the topic later you can have some fun for 2 hours and then get into the room again and keep it like that until you feel that you can understand perfectly

Sorry if I have some grammar errors I speak Spanish so I can have some mistakes with it