You sound very much like me a couple of years ago
I had a very similar experience at the beginning of my relationship with Z. Two differences: we were planning to live together rather than getting married and she wasn't expecting a ring on her finger as a possible outcome.
We too had discussed the idea of polyamory intellectually but I was totally blown away by the painful emotional intensity that erupted from inside of me when he actually went away to be with her. And they weren't even sexual.
In the couple of years we have been together I have experienced the deepest most loving relationship of my life but we have also bounced backwards and forwards between him trying to give her up and me trying to accept polyamory.
This forum has been a really positive thing because I am actually seeing poly now as a positive for our life rather than a necessary evil that I just have to earn to live with.
It sounds as if things have got better for you but it won't be an easy steady path, you will still have emotional highs and lows when you seriously wonder whether it is all worth it.
We have had Z's secondary to stay and we have stayed with her but to this point she has found it very difficult to be the same around him when I am around. This is our next step. Accepting being a secondary in someone's life is also a very difficult thing. One thing in poly, life is rarely boring.