I was only able to make the decision to leave my drug-of-choice ('cos that's what my girlfriend was, truth be told) when I finally realized that the pain of being without her--as excruciating as it would be...and it was--would eventually fade, while the pain of remaining in that toxic relationship would only grow and get worse with each day/week/month/year...
I had to put some very strict boundaries in place in order to begin a slow and difficult healing process. We'd been together the better (or worse?) part of ten years, so there really were no shortcuts to healing.
The first boundary was distance--the distance from Florida to New York, where she went back to live when we split up. The second boundary, which I put into place grudgingly, was cutting off all contact and changing my phone number to an unlisted one. You see, my requests that she stop calling me weren't working, so I had to make myself kind of invisible.
Another boundary was with myself. I chose to limit my thoughts where she was concerned. Rather than fall into romanticizing "maybe it wasn't that bad?" or "how good it could be if only...", when these thoughts arose, I chased them away as best I could. I literally put time limits on how long I'd sit in those mental places, 15 minutes max, then shorter and shorter.
I was told that my Higher Power would do for me that which I could not do for myself, so I turned to that Power through prayer and asked often to be relieved of the obsession, as it did not serve me or anyone else anymore (never had, actually).
I guess the bottom line is that I recovered through time and positive action. It took me a long time to get as deeply sick as I did in that relationship; it was gonna take a long time to heal. And it has.
I think we all find our way if we're looking...