That's ok, but not quite enough. If I were you, I would tell him/her the reasons for that. That's how you're going to prove him/her that you really appreciate his/her force and time. If you tell him/her your reasons you're going to help him/her not to repeat the same mistakes as before and to be better teacher for the future students + you're going to keep your nice and friendly relationship and that's the most important :) I hope I helped you. Please, let me know if there's anything else I can help you. Have a nice day.

You don't have to be so formal with this type of thing. You're not obligated to stay in his/her class. Are you asking how to persuade a person that you really appreciate what they've done for you? If that's the case, then I'd say keep what you wrote plus at least 1 acceptable excuse as to why you're leaving. I doubt that a college professor will be very offended about not being able to teach you because it's not your fault. You just learn differently.