Ouch. I don't even have the words. I'd be hurt, angry, sad. Beyond that, I don't know. I guess I would have to decide if the trust could be rebuilt. And, if it could, keep the relationship mono until it was. I'm so sorry you're hurting. But Mono is probably right. This is something you'll have to work out for yourself in the end.
I'd rather be hated for who I am than loved for who I am not.