Hi, MCow. Welcome to the forum.
I am so sorry for the situation you are in. I can tell you are in a great deal of pain. My heart goes out to you.
The good news is: you're not alone, and you've found a good place to land here. You are by no means the first person to come to this forum with this kind of situation. While "my spouse wants to have an additional lover" would be a real jaw-dropper for most folks, around here . . . not so much.
So, in general, I would encourage you to look around the forum, and especially the "New to Poly"and find the other threads where ppl are working through issues like yours. It will help you feel less alone and adrift, and help give you some perspective. Plus, knowing how others are dealing with similar struggles can help you get a handle on yours. And you'll come across some terrific resources, strategies and advice as well.
I'm sure there will be wiser and more experienced poly folk who will help with better advice and insight. I mainly wanted to say Welcome, and I'm sorry you're hurting so. Good luck to you as you work through this.