Doubts About Being Non-Monogamous

So, I (F35) was with my partner (M35) for 15 years. We were open for around 13y but very much in a constrained manner. The expectation was for us to always be primary/nesting partners and close the relationship if things ever got complicated, messy, or if one of us was in need of that security. Things weren't perfect, I dealt with a lot of jealousy, but from my understanding we were both well and happy. We always communicated openly and with love and understanding.

I suppose we were not both well and happy after all. Very long story short, once a specific partner entered into his life, the way our relationship always worked was not enough for him anymore and he was adamant that it was hurting him. That he was poly and he specifically wanted to have long-term relationships with other people. Now, although I respect that choice, it was not how things had been discussed in the past. I never wanted that, and even less so when we were planning on having babies soon. I always wanted to close the relationship during pregnancy / first years of the babies' lives and I didn't want to raise them in a poly environment, especially while they were small (no disrespect, really, it's just my preference). This was a no-go for him. I tried to be ok with it given how much I loved him and how important this relationship was to me, but this past year (all while grieving my father's death) it honestly destroyed my sense of security in the relationship and my self esteem - and I ended up in a severe mental health crisis. He very much had the approach that I wasn't ok with it because I was insecure and that if I just grew enough, if I worked on myself enough, I'd see that this was the best way to go. It doesn't help that his partner has kids of her own and he's very much a part of the kid's lives - to him it's a beautiful reality.

About a month ago things got unsustainable and I broke things off. It's impossible for me to be in a non-mono relationship like this, especially because having kids is crucially important to him. I am going to therapy and trying to make sense of things. This whole thing did get me doubting my own non-monogamy, though. Has anyone been through something similar? I can't imagine myself ever being ok with being open again, but I don't know if this is just a normal reaction to a horrible and life-changing break-up or if I found something about myself.

Any advice would be greatly appreciated. Thank you.