Need advice on how to approach a conversation with a long term partner about their potential new relationship

It's funny, but I already wrote a very detailed post about my struggles yesterday but ended up not submitting it, which is a good thing because the situation became a bit clearer since then. I'll try to be brief but can't promise :D

My partner and I are both women in our 30s. We've been very close (like "partner level" close) for more than 5 years but "officially together" for 2. We made a rookie mistake of not discussing poly ground rules/boundaries in the beginning (and we still haven't done that), but we vaguely agreed on ENM when we got together.

Before we got together, she had some romantic tension with another girl Kylie. They didn't discuss it back then but my partner talked to me about her feelings and how she felt like Kylie had feelings for her as well. When we got together with my partner, she claimed she didn't want to pursue a relationship with Kylie. One of the reasons was that Kylie didn't know an essential piece of information about my partner.

Recently, my partner opened up to Kylie, and Kylie took it well. They met up last weekend (all three of us are long distance). The original reason for the meeting from my partner's side was "to have closure", but they ended up hooking up, and now their relationship status is up in the air.

My partner reassured me multiple times after that that she sees her future with me and she loves me and she doesn't want to lose me. She feels extremely bad for saying one thing and then doing another.

I honestly don't know what to think or feel. I'm not a jealous person. I had poly relationships before. I explicitly gave my partner a green light to be in a relationship with Kylie and I did it multiple times. But this switch ("no I don't want it" - "um you know we ended up hooking up") makes me lose a tiny fraction of trust. I can't know this 100%, but I kinda think that if she told me from the beginning "hey we will spend the weekend together to see how our relationship can progress and we might want to be intimate", I would be totally fine with it. But she didn't, and I'm a bit hurt now. I don't think it was malicious - I think my partner truly didn't expect this to happen, and I do believe she wouldn't do anything if she didn't have a green light from me. But still... Two things can be true - I can see her side and I can have my own feelings about it too.

My question to the community is the following. Tomorrow, we are planning to have a discussion about the whole thing. And I want to know what bases I should cover. What questions I should ask her. What questions I should ask myself beforehand. Is it worth bringing up my hurt feelings knowing that my partner has a tendency to beat herself up mentally even over smallest things? She already feels bad... And I'm not sure I would gain anything by talking about it.

So far, I can only think about one EXTREMELY important point to bring up. I'm really afraid to hear the answer, so I haven't yet asked if Kylie knows about me... Because if she doesn't... I don't know if I can ignore this. My moral compass obviously says I can't, but there's also a part of me that is preemptively ready to excuse this by saying "well it's her first poly relationship, she's allowed some mistakes". To be perfectly clear, I DON'T KNOW if my partner told Kylie about me, I sure hope she did, I'm just trying to prepare myself for every outcome. So I would appreciate some guidance on this as well.

Thanks in advance everyone!

Just in case - I'm going to try to go to sleep now, so I will read your responses (if any) in the morning (I'm in Europe). This is a throwaway account, also JFYI.

Edit: replaced a fake initial K with fake name Kylie according to auto moderation. Sorry, I never posted here before :)