Whimsey, I am in a similar situation and my heart goes out to you. My husband has asked me to accept his relationship with his girlfriend and it has been extremely painful, to say the least. I am slowly starting to open to the idea. What has been helpful for me is developing a friendship with his girlfriend, which helps me to feel less left out. I don't know if this is an option for you, but if it is, it might be worth trying.
My husband's relationship went on for several months before he told me about it, and the pain of being cheated on and betrayed will take a long time to heal, and is actually a separate issue from whether or not I can accept a poly relationship. So I would just advise you to do all the grieving you need to, let yourself feel everything you feel.....and as you slowly heal from this hurt, maybe it will begin to become clear whether or not this is something you can accept.
My therapist said something interesting - that my husband wanting to be poly is similar to if we had both been ardent Christians and then one day he suddenly told me he wanted us to be Muslims, or pagans, instead. It's that big of a change in belief system, and you can't make such a big change overnight. Spending time with people who share the same belief system can help you make that change, so I've been reading these forums a lot as a way of easing into the idea.
Here is a link to the thread I posted when I first joined. The responses I got were enormously helpful and maybe some of them could be helpful to you as well. Hugs and best wishes to you!