I can only really reiterate what everyone else has said to be honest. I've been in a similar situation and went back thinking it would be better but it just got worse. By staying or going back to them it's like validating what they have done so they don't see any issues in doing it again. Everyone is different though so nobody here can really say he won't change, I think we're all just doubtful.

The decision is totally up to you but you're very young so although 4 years will be lost you can still have a lot of happy years with someone else. I think that's better than feeling the way you do now for the rest of your life.