Hi Mahogany, I just want to give you a little encouragement. I'm in the same situation, only it has been 6 months since my husband told me about his girlfriend. In the beginning I felt just as you do now and the pain was unbearable. But so much has changed: she has become my dear friend and I no longer feel threatened by her relationship with my husband, because I can see that it has actually improved our marriage (strange as that sounds).
My advice: first of all let yourself experience all the pain you are feeling; it is normal and you just have to go through it. It took several months for me. Keep communicating with both of them. Spend some time reading on here and learning more about poly and how people deal with jealousy. Don't expect to overcome jealousy and accept poly overnight; it is a huge change for you and will take time to get used to. But it can definitely be done