There is no way I could say anything to ease what you are feeling at the moment. That being said, remember that you are an individual and have strenght and independence and a right to be loved the way you want to be. It sounds like this has happened very recently and that usually means that you can barely focus on anything but pain. There is positive possibilities in this though. There is the opportunity to open your own heart up to others if you wish, to explore others with the support of a partner who should understand. I know this sounds really weak coming from me but there can be benefits and believe it or not a possible deepening of your relationship as well.
Understand that regardless of anything else, you are not in an inexscapable trap as you may feel. The idea of sharing him is probably as hurtful as the idea of him "faking" monogamy. There are options available for both of you to be healthy. Both of you deserve to be happy, loved and healthy how you want/need to be. Do not avoid this and talk. Cry scream or whatever but both of you need to be honest and strong. And remember that he is in pain too.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over