I [26/F] am still obsessed with an ex [34/M] from a few years ago. I don't know why I haven't gotten over it.

The title pretty much explains it. I dated Ben* for about a year from 2014 to 2015. We had a horrible, unhealthy relationship. He was manipulative, emotionally abusive, and a cheater. I stuck with him for entirely too long because I was rebounding from my first serious relationship at the time and in a pretty strange place emotionally. We eventually broke up for good when he ditched me for another girl and ghosted on me.

For a while I still contacted him pretty frequently, either trying to get him back or just to say nasty things to him out of anger. He'd respond sometimes, and we'd usually just end up getting into an argument. Obviously none of that helped me move on, so I eventually cut Ben out of my life altogether and started dating someone else.

Fast forward to now, I'm still in a happy relationship with the new guy and I don't have any romantic feelings left for Ben. But I still think about Ben all the time, and I don't know why. I just obsessively replay all the negative things that happened between us, and sometimes find myself wanting to talk to him and just ask him why he treated me that way. I don't understand why it's still bothering me so much.

Has anyone else been through this? Advice?

tl;dr: I can't stop thinking about my awful ex, and I don't know why our history is still affecting me so strongly.