HELP! How do you tell your bf this??

I’ve been dating my boyfriend for a few months now. Everything has been great, and he seems like a wonderful guy. A few days ago we were talking about what kind of things turned us on, and my input to the conversation was I like dominant guys. Yesterday, I saw him and he was SO rough!😬😬 He has never done this before, he left bruises from grabbing me so hard and also hurt me down there. He felt horrible after I told him he hurt me. But how do you tell your partner you aren’t into that nicely?