Sometimes it feels like you're not a person with a disability, you're a disability with a person, right? I hate that, too.
People without disabilities have the same problem. Others tell them that they can't do something or they shouldn't do something. You can't be a model, you're not pretty enough. You can't be an engineer, you're not smart enough. You can't be a black belt in karate, you're not a boy. You can't....
Confidence comes from one place, inside of yourself. You can do stuff. You just did, right? It's harder to listen to you when others are saying the opposite. I don't know why, but it's easier to believe badly about yourself than the good things. Most everyone has confidence issues.
It might help to go back through your notes, especially in the classes that were hard, but you did well in anyway. Look at what you did, by yourself. It helps me. It's one reason I love math. I can whip out the old textbooks and work a few problems. There's always hanging out with people who appreciate your abilities thing, too. I get told to do that whenever I start whining about how I can't drive.