"Oh, look who's back." he says with a smirk, leaning back against the wall "I knew you'd come back to my feet. You always do."

He's right. That's how you two work. He cheats on you and when you find out, you break up with him. But you always end up going back to him: either because he persuaded you and by mere choice of your own. You just can't help yourself: you're trapped in an endless cycle.