How to deal with my Mom and her Ex?

My mom and her ex have been off and on again for 15 years and they share a child together. The last time they broke up it was a 3 year battle with him. He stopped paying child support and I had to move home to help her with the rent. She took him to court which was long and drawn out and very stressful for the whole family. My mom was constantly depressed and I was very worried about her. She was at my house every day crying or angry. I was constantly going to the courthouse to file papers for her or serving him. Everyone was helping her. Some people even helped her out with money so she could hire a lawyer to fight him. He lied about his income, he lies about everything. I can't even begin to tell you all of the stuff he has done to my mom. He is the most ignorant man ever. Please remember that this is not the first time we have all been through this. This has been going on for 15 years. My sister and I have had to take turns staying with her because we were afraid of what she would do if she was alone. Finally the court battle is over and he is out of our lives. Up until 7 months ago she swore he was still stalking her. Now all of a sudden they are friends again. He still has a girlfriend that I'm sure doesn't know he is taking my mom out for lunches. Still a dog! When I confronted my mom about what she was doing she told me I was over reacting. I know I wan't super diplomatic about it but I told her that she put us all through the ringer when they were fighting but now we are all expected to be happy about their reunion and he hasn't changed at all and that I will never forgive him for what he has done. I can't stop her from seeing him but I don't want to know about any of it. She can keep it all to herself because he will never consume my life like he did the last time. She then told me that I was over reacting and needed to get help. I'm really not sure what to do anymore. I feel like I can't even look at her let alone talk to her. Everytime I say anything to her it all gets turned around on me. I'm always the problem. At this point I don't want anything to do with her. It really hurts me to say that but I'm tired of everything being my fault. What should I do?