Telling mom about abusive ex bf

My mom keeps bringing up my ex boyfriend. I think she wants me to get back with him. He can across as a great guy and out families got along great! On the downside he was abusive and I never told my mom. I ended it 2 years ago and haven't really dated since. She wants me to get back with him. How do I tell her now? I want her to understand why I can't get back with him.