As a fellow music major, I have to say that his actions apall me. I'm going through some tough times and I'm not up to par on a lot of my skills. But my professors recognize that I'm trying and they're going out of their way to help me and give me good feedback. Your instructor should absolutely NOT be able to talk to you like that, in private or in front of others. If I were you, I would report him to his department head immediatly. If they are a decent person they will take some action and at least talk to him. It doesn't matter whether or not you have a disability, you are a student and he is the teacher. His job is to encourage students no matter what, not put them down. If I was his supervisor, he'd be fired.