How to cut off my mom?

Me and my mom have always had a bad relationship. It was at its worst when I had to move in my best friend when I was 15. My mom was dating a guy and he was always an asshole to made and made it clear he did not like me. He was arguing with me when I was 15 and I yelled at him to get the fuck out of my room. He got mad that I cussed at him and slammed the giant mirror that was the last gift I got from my dad before he died on the ground and shattered it. Then he grabbed me by my hair and dragged me through this class and threw me through a wall. I was able to run out the house and called my best friend to get me. My mom made a decision and chose him over me. I'm 26 now and I found out my mom is BACK with him. She said it happened a long time ago and everything happens for a reason. I asked what reason did he have to throw your 15 year olr daughter threw a wall. My best friends dad was picking glass out of my feet. My mom started saying she deserves to be happy and I can't expect her to do what makes me happy and kept saying everything happens for a reason. I'm done. Our relationship has never been good because she has always failed to ever put me first. If she wants to be with him... A man who abused me. Fine. I'm done. I just need to know how to cut her off