It seems to me that you've already got one foot out the door.
And it may be that the healthiest thing for you both, is to end the marriage. She's mono, you're poly... its a huge difficulty to overcome.
Anyhow, I've noticed a common thread among the couples who seem to be working out this dilemma/negotiation in a healthy way. They seem still deeply in love with their primary.
I think that most mono's in this situation can't easily wrap their minds around the idea that you being poly is NOT ABOUT THEM. They need tons of love and support and affection to feel safe. Because their gut is telling them, if you were really happy with them, you wouldn't need anybody else. So they need all the evidence to the contrary that you can muster.
It sounds like your relationship is past the point where you can honestly give her that. You sound way more frustrated than in love.
Anyhow, best of luck finding a way for both of you to be healthy... together or not. And echoing what Mono said, the MOST important piece is the kids. If you break up, you'll still be co-parenting, I expect? If so, a healthy ongoing working relationship is key. And the more loved and respected you can make her feel during the breakup, the easier that working relationship will be.