I did it twenty years ago with a crazy, but then-attractive woman who I was seeing. I found out she had been sleeping around on me the nights I was working or otherwise busy. Big scene. I found her at his apt. Told them both to go f^&% themselves. I went back to my apt. about 4 AM, couldn't sleep, saw a then-friend of mine, a local cop on night duty driving around, told him about it. He told me, stay pissed, and determined, so she won't be able to get her hooks back into you. I was pissed for weeks, then I saw her. Like the young fool I was, I started talking with her when she came over and said she was sorry, bla bla, we should give it another chance bla bla....
If I could ever go back in time, I would go back to there and find some way of keeping her away from me, ever.
We got back together and ended up living together, but it was never the same again. Truly the worst time in my life. A rollercoaster, everyday, sometimes every minute. Only going to grad school, and then getting an all-expenses-paid scholarship to study in Germany saved my ass and my mind.
Haven't seen her in almost two decades. One day, just for ha-has, I clicked on a match.com email I rec'd, just to see what's out there. Hell- there she was, and other than about twenty more years of hard living and twenty more pounds, she still looks and presents herself the same [cringe]. She still lives in MA, is divorced (God knows how many times), has kid[s], and looks like a freight train that took a dirt road. Time wounds all heels, I guess.
Thank God I did not marry her, let alone start a family with her. She was never happy with herself, or anything, really, but I was to young, too blind, too inexperienced to see it. And I just about paid for my mistake with my nervous system....
So, in my experience, no, it's not worth going back. There are PLENTY of good women out there-there is no sense settling for somebody, just because it's familiar and easy.