The only thing I can really offer, is what is he getting from the OW that he was not getting before? Have you explored that? It may be that its not something more at all, just different. In that case there isn't a whole lot you can do. Keep working on being the best person you can possibly be and make a life for yourself. If he continues to see you waiting for him, there isn't really much incentive to leave the OW.
Stand up for what is right, even if you are standing alone.
