I feel like I usually hear this story from the point of view of a guy. He loved his wife but was curious about sec with other people, so he pushed for an open marriage. After he got his itch met he wants to go back to monogamy, meanwhile she has found someone she’s really into.
Nice to know it happens the other way around.
As a kitchen table polyamorous woman, all of this seems crazy. So you wanted monogamy, partially because you didn’t feel like sleeping with others, and he wanted to keep going with this other woman. Why couldn’t you just stop dating but allowing him to date others? And if he wanted to stay with her part time, so what? Then you get some alone time in the house! Who doesn’t want their husband out of the house every now and then? Well, my partner works from home so I get tired of her being here ALL THE TIME! But maybe you already don’t get enough time with him?
I mean as long as he is still pulling his fare share of childcare and house duties, so what? It is possible for just one person to be poly saturated with one partner while their partner has others. Was the idea that he was with another woman but you didn’t want to be with other men at the same time made the whole thing uneven?

I guess I don’t understand, but that’s probably because my situation is so different. I find my partner to be very intense, and she’s home constantly. I
Love it when goes and spends time with my meta. I love the peace and quiet to myself. She still is even with childcare and housework. Also my meta has a total of 7 partners, so they aren’t that demanding of her. Their needs are spread out by 7 different people.
I personally don’t feel like having partners at the moment, as you say life is busy and I don’t have a lot of time between our child, work, exercise and hobbies. But I may one day.

It sounds like non-monogamy to you was about sex, sexual exploration once you got that, you wanted to close things without regard to the fact your husband may have fallen in love with your meta. It feels rather self-centered and inconsiderate of the other people involved, like your husband and meta, and their feelings.