First lemme start with *HUG*. I'm really sorry you're going through that kind of torment. And from what I read, you definitely sound tormented. In reading, one thing came to mind over and over again. Why aren't you two talking about this?? I know that you said that there is some history that you didn't care to rehash, but if she's attempting to start a poly lifestyle, then shouldn't that be something that she's very open and honest with you about? Just like you need to be COMPLETELY open and honest about what you're feeling.
How long have you two been married? Has this topic ever come up before?
I can't say that your particular situation is going to be ok, as I don't know all the details. But what I can tell you is that this lifestyle is VERY possible when you have the right people involved. I say that because this life ISN'T for everyone and that's ok. Some people are wired for only being monogamous and there is absolutely nothing wrong with that. This won't work if the issue is forced, ya know?
There are lots of wonderful people on this site that I'm sure will be able to offer excellent advice and words of wisdom. I hope you can find what you're looking for and don't beat yourself up because you can't wrap your brain around this yet. It's ok!!!
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!