Conversation about being poly / vs agreeing to be monogamous

My partner and I have been together for 6 years. There have been conversations about her being polyamorous. But nothing was ever determined. My partner started talking to this girl online. She wrote she wrote her a love letter. She showed me the love letter. I was hurt. I was not expecting it. In the beginning of this she would tell me they were just friends. Best friends and even tho they were romantic and fantasying with each other she would tell me that’s what friends do. I would argue back by that is not what friends do. And she’d say “ by my definition of friends” it led to me breaking up with her. During that week she got with said “friend” because there was no reason to have limitations. Well, I came back because I wanted to fight for this relationship.

Feels like I came back. And gave up my right to be hurt.
I don’t know what healthy polyamory looks like. I have been in a polyamorous relationship before I can see where it can be beautiful.

There was a lot more within that that happened. But after months of conflict from this I feel a deep sense of betrayal. I don’t feel like I can trust her. I don’t feel like I can hear her side because it feels so manipulative.I told her many times that there is work that we
Need to address in our relationship before having other partners. Well long story short she ended up with 2 online partners and I kept feeling panicked and not okay. I have tried many times to be okay. But it just hurt. The way she would treat her online partners better in many different ways. Just hurt. It was hard not to compare myself to them when they received such vastly different amounts of care and love. I told her that it’s not normal to have 2 girl friends while we are not okay. She’d assured me that she didn’t need to tell me that “ Normal” polyamory does not mean that she needs to tell me how many partners she will be having.

Well now after so many efforts and pain and heartbreak for months. I keep feeling like she cheated and I was just naive. I freaked out and had a full blown meltdown breakdown from not knowing what was real not knowing who she talking to not knowing what to trust. She was detached and felt like she was minimizing my experience. I can’t say what I did but it was not okay. I scared her and myself. The relationship had gotten to this level of bad. I can’t take back the pain I caused. I am aware how damaging this was.

As awful as what I did was I just feel like it’s now the subject instead of the months pushing to that breaking point. The desperation that I felt that i needed her to show some sign that I matter. Again I’m jnot excusing my lash out. I didn’t physically hurt her but I know I caused emotional damage. Again I understand that. I recognize that.

One thing we always come back to is Claiming she didn't "cheat" because she never explicitly agreed to be monogamous. I feel like she did. I feel like a lot of my trust has been eroded.

I am not looking to hear that we should break up. I am looking for just support in this. And I guess external validation.