He can’t understand the teacher’s approach. He doesn’t like the teacher ordering him to do things he doesn’t like. He has been repeatedly admonishing himself "Never let others tell you what you can and can’t do."
He couldn't understand what the teacher was doing, he didn't like the teacher ordering him to do something he didn't like, and he kept telling himself, "Never let anyone tell you what you can and can't do."
He can't understand the teacher's way of doing things. He doesn't like that the teacher orders him to do things he doesn't like. He keeps telling himself "never let others tell you what you can do and what you can't do",