What do i do in this situation.?

I am in a relationship with a guy that has one child (little girl) and another one on the way with his ex (by a same woman) and I dont have any children-- During the time she conceived he and i were together but going through a lot of problems; the problems he was causing because he was obviously wanting to be with her but did not know how to let go of me. He is a type that if he does not have you he will want you, as soon as he sees that im drifting away he will start trying with me. We broke up too many times to even count. His ex is just nothing, she depends on him for everything, she is in love with him, wants him, and always tries because she has his kids eventho i am here. He says he is done with her and wants to make things right with me. I have been trying to let go of him so many times, but he is a good manipulator and always gets to me because i do love him, I did try leaving him but he just never lets me move on. I even tried telling him nicely that i cannot handle his situation, all he thinks is that im breaking up because i want someone else, and that is not the case. I would never do that, I am just too stressed and hurt to handle this, but I love him and now Im here again with him. Am i an idiot for being in the picture still? We have not met each others family either. Im just hurt thinking that he will leave me for her again and just leave me heartbroken, but if i try breaking up with him again he will not take me seriously since ive done it so many times and managed to take him back every time.what do i do???