If I'm not mistaken, you've shared your own history of an abusive relationship. And if I'm wrong, my profuse apologies. If I'm correct though...
Go back to the first time you knew you had to leave. Not the time you really did...but when the first thought appeared. Maybe, like me, you did even separate. Now, remember the hook that drew you back. For me, it was being manipulated with my great desire to have two parents raising kiddos - promises of change and counseling at the ready. As I became more determined to leave the relationship, his manipulations would change to fit the circumstances - he'd purposely prey on my worst fear, like losing custody, informing my family how awful I was, etc.
Despite violence, rages, weapons, and near-daily threats I went back 4 times in a total of ten years before leaving permanently.
In the absolute worst part of it, when psychological abuse had evolved into physical threats which evolved into sexual abuse, I was telling everybody we were going to renew our vows. I was at my absolute lowest but I was so desperately attempting to hold onto the relationship because I'd been taught (read manipulated) to believe I couldn't survive without him. Literally.
I left a month later and everyone was shocked. After all, we were supposed to be renewing our vows.