Of course you are grieving. You're grieving a very real loss. The loss of your relationship with your husband as you have known it. It's no one's fault but it is a loss. And you don't know what's coming, can't know what's coming. So there is fear and uncertainty too.
It's totally ok to feel these things. Many people do. Your husband, in the corner of his brain not taken up with new love and NRE, may feel similarly for his relationship with you. But even if he doesn't your feelings are what they are. If you don't feel them and move through and with them, they will stick with you and possibly poison your future. Feel the grief, acknowledge it and give it time to sit with you, and you with it. You've already felt the possibilities for joy in poly - this is the flip side and also totally normal.
You're not alone.