I think you should start slowly. Try to recognize the feeling that you start to get when you feel like talking down to someone and try to connect that with what they said or how they said it and then try to stop yourself. Do it slowly, maybe even tell your friend what's going on, that you say mean things because you don't feel good about yourself and it's just the automatic response you have to certain situations, i'm sure they'd be willing to understand and help. You should reach out to a T, the right one would be helpful.