Maybe an alter of your dad was created so you could finally stick up for yourself and tell him exactly how you feel about him. This might be your time to truly get your chance to tell him everything you've ever wanted to say to him. Your other alters may get their strength to stick up for themselves against him once they see you are strong enough to confront him.
You asked how you can stick up for yourself and not take what he is saying to heart...well, I would say that you are stronger than him because you are not like him. You chose to realize the things your dad did was wrong and you didn't want to hurt other people the way he hurt you. Don't let the alter of your dad make you doubt yourself. I'm sure you can list many things to him that he has done to hurt you. You have the upper hand against him because you are stronger and better than he is as a person. He doesn't deserve your fear; he deserves to fear you. You know the kind of person he truly is and you know you are a better person than he is.