Most Helpful Girl
Up to my knowledge, it is because you never closed the cycle, even though you've tried, you keep thinking that it may no have been his fault so you keep giving him chances because "maybe this time would be different" you still like him cause he hasn't give you a chance to have closure, and he keeps calling you because he knows you'd take him back, he knows if he doesn't give you the chance to close your cycle you will still be there and go back whenever HE feels like it.
I really recommend you, for the sake of your emotional health to stop answering when ever he calls, just say you are busy or something. I can tell you first hand that this kind of relationships are really exhausting and draining. If you keep answering he'll keep calling.
Hang out with friends, meet new people, keep him off of your mind.