I have been your boyfriend (a female version). Does that make sense? I left an unhappy marriage, found my soulmate and also found him to be polyamorous (even when he didn't realize he was), so we've addressed many of the issues you're looking at. It's been a two year journey and at times during that journey I would have agreed with Red Pepper about both going for what you really want.
We've stuck it out and we're winning!! I agree with RP in that it is important not to compromise what's important to you long term but that is actually a really interesting concept. The more I drilled down the more simple my goals became. I wanted a passionate, beautiful, healthy, honest relationship. I had all those things and it seemed wrong to "send it back" just because Z is poly. The poly component, when looked at in relationship to everything else was small, it was my reaction to it that was big.
So together, we worked me through it. While hanging out on this forum and the mono/poly yahoo group I found that this is quite a common issue and I've started a blog www.polyamorouspeople.typepad.com
about how we worked through it.
So have a look on that and then if I can be of any specific help let me know.;