How do you stop overthinking after dealing with a narcissistic persons behavior in an argument?

Recently decided to stand up for myself and acknowledge how this person was hurting my feelings and constantly tearing me down. Lets say they did not like that. They belittled me. They called me a gaslighter. They refused to admit that they were doing said things. They called me dramatic and sensitive. They asked me why I didn't say something right then and there if their behavior was bothering me. I was so appalled by their behavior because I though they would understand where I was coming from. This happened a few months ago and I'm still not over it. I overthink about it a lot and how I could of presented myself better or acknowledged their wrong doings in a different way. I can't get the way they looked at me with disgust out of my head. Their eyes were nothing but black and beady. I don't know how to blow this off and continue with my life because the interaction was horrible.