Originally Posted by SeventhCrow
I'll also hallucinate that he expected to not have to stay in contact with you when off with her.
sounds like quite the hallucination you're having
I think you've been very supportive given that you both entered the relationship with a monogamous understanding. We're not perfect and I think the fact that you feel ashamed for reacting the way you did and (I assume) have told him that and apologized to him speaks miles. I think in the end he will be willing to work things out with you, and come to a better understanding with you of what you each expect and are willing to offer.
If he breaks up with you over some hurtful things you said once you've apologized, and expresses an unwillingness to try to work things out with you, then it could be that he is coming to find that he doesn't really feel the way he expressed to you he did upon entering the relationship with the other girl, because, at least from my perspective, if he truly loves you wholeheartedly he will try to make things work with you if you match him in effort.