What I dont understand is why bring me back here if he still didnt know who he wanted to be with???
Because he's a pig and wants you both.
No the relationship is not fixable. Go to your moms and go no contact with him, get an atty, whatever you need to do.
He will never change. If he ever does give the OW up, he'll just find someone new at some point. Your whole life you will never trust him.