I need help, I want to leave mentally but I can’t

I want to break this relationship off, we’ve been together for 6 years and he has cheated on me even slept with a girl and tried to sleep with his ex. We have a 3 year old son, I want to leave him I do my heart tells me too but mentally I can’t. I can’t deal with the hurt, I know he’ll act like he doesn’t care and will move on he knows how to get me. I want to leave mentally before I leave physically any advice?😔