Most Helpful Guy
It is not easy to figure out someone's problem through some text.
Yet, I will give it a shot anyway.
I have seen this behavior before. One of the main reason you get back with your exes is because you are a trusting person and desperately want to feel wanted/loved. As soon as a guy does something horrible like that and leaves, you feel horrible, obviously. You resent that feeling like anyone would, but you take it to the next level. You just want things to go back to the way they used to be, before all that bad stuff happened. That is why you allow yourself to get back with your exes. You trust the guy to chance while you hope that he will do better this time.
The cold hard truth is: he won't. If a guy cheats on you or abuses you and leaves, he is not going to change when he comes back. Guys who are genuinely sorry about what they did are much more likely to change, but it still isn't a guarantee.
You have to set boundaries. As soon as a guy does something horrible like that and there is no prospect of him changing or stopping his behavior , you have to force yourself to leave and stay away. I really think that you should cut off the "friend with benefits" thing with your ex. It will eventually cause a lot of emotional pain for one of you, if not both. It will only remind the two of you of the feelings you had for each other. Since you have broken up twice already, there is really no hope for that realtionship.
Move on. When you meet someone new, give him a chance, but respect yourself too. Do not be afraid to tell him when something is bothering you.
If you have more questions, feel free to ask them0