I am very happy to report that things are much, much better now.
1. My wife understood that whatever work we need to do on this, we have to do it together. She can't just ask me "stop being poly ... come back to me when you're ready". It's a team trying to work through change.
2. For the time being, I'm cooling off relations with the new girl. I refused to agree to anything like "I will never talk to her again", but it is very good for my wife and I to do this for the time being, and so I happily cooperate.
3. We have started couple's therapy. It will take time because we have a lot to discuss, but we have already gained some valuable tools.
4. I think already in the short weeks that passed, she is slightly softening up. One night I defined us as "two polys that just choose to keep the number of people in our group to two for now", and she didn't flinch. I am reading into this that maybe, sometime in the far future, she'll be willing to consider ... some form of poly. Maybe it will be only sex, maybe more, but she's not saying things like "I'll never be poly" anymore.
5. She understands that she is more important to me, right now, than being poly. She still has issues with the "right now" part ... she's afraid I'll change my mind in the future ... and I can't promise her that I won't. We'll work on it.
The last few weeks have been an emotional rollercoaster, but I'm happy we've had that - whether we stay together or not, we'll be much stronger and smarter now. We've identified some core issues in our relationship that has nothing to do with poly, are working on them, and are doing it as a team.