I am seeing a girl who is ruining my life. But I cant stop loving her, an letting her back in, how do i stop?

What do I do. She says she loves me too. But she comes into my life, makes me believe in us, then just stops. She soes once a month. I have been dating a lot of other girls, but it always ends because I cant get over her. I don't know what to do. She is horrible for me. Everyome i know tells me to stop with her. I know i should. But I cant help that I love her and i want things to work out. We got engaged after being together for 2 months, we were really in love, infatuated. Then she screwed her ex a week after and i took the ring back. I really thought that would be the end. it wasn't She got pregnent, and didn't know if the father was me or her ex. She had to get an abotion cause of health reasons. I ended being so stressed out i had a seizure and hit my head and was out of work and school for a month. I though that would be the end of it. It wasn't. I can honestly say that even though i love her sooo much, I hate her at the same time for puting me through what she does. What can i do to stop seeing her. Please don't just say "stop" its not that easy. I need real advice.