This is an old post but well I'm here
we close down the relationship because she doesnt feel secure enough, which was also a completely valid request
No it wasn't. Her options there were leave the relationship or seek therapy. Asking you to change the relationship model to something you don't want is a reach. Rookie mistake I understand how it happened. But there's your problem. This person can't or won't do non monogamy. If you want non monogamy, leave.
It's fine if she doesn't want to date anyone else. I know a couple where the one of them doesn't, and the other has a sprawling polycule. They're happy
It's not fine if she wants to control what you do. She will never be secure like that and you will be miserable.
I dated someone who was very insecure and controlling. It was horrible. Nothing was ever enough and their metamours suffered.
Eventually they had some breakthroughs in therapy and now are extremely poly, but they had to get there themselves. Trying to control other people to make themselves feel secure was doomed.