How do I tell her this?

I’m 18 and live by myself. I have a physically and verbally abusive stepfather who I’ve called 911 on but he’s never been charged due to lack of proof or him claiming I’m framing him. In addition to the episodes of abuse, he’s overall a manipulative person who calls me a little fucker, a slut, etc. I moved out and cut him off in September. My mother has been asking me for months to coms visit them and “reconcile” with him. I love my mother despite the fact she stays with him, so I told her I would reconcile a relationship with him if he went to therapy. On second thought, I don’t think I want a relationship with him at all. She asked me to stay with them for Christmas and I don’t feel safe doing so. How do I tell her I changed my mind and don’t want a relationship with him? How do I “go back” on my word when I’ve already agreed to it? I regret ever saying that because by doing so I betrayed myself and my boundaries, and I think I was just feeling bad for him in that moment.