To breakup or not to break up

Problem/Goal:

My question is I don't know if I should break up permanently. Maybe my hesitation is because of the sunk cost fallacy or a fear of abandonment. But at the same time, my partner has a lot of positive qualities to offer as well.

Context:

Both in our mid-20s.

​We had a really big fight. My partner accessed my past conversations with a fling from three years ago. I never had any type of serious romantic relationship with this girl.

​My partner is upset that I never fully talked about her. My partner knows that we still have some kind of contact from time to time, maybe at least once a month. It is just a friendly conversation. The only detail that I revealed to my partner is that my ex-fling already has a fiancé, and I even talked to my ex-fling to establish the fact that I wouldn't be comfortable hanging out with her knowing that she has a boyfriend.

​My partner and I both have access to each other's phones. My partner dug up our past conversations, and she is upset that I never told our story in depth. I never mentioned the full story because I can barely remember the details.

​The argument spiraled, and now we have agreed on a cool-off period. By the end of the argument, we were both relatively calm, and we both apologized.

​I do not genuinely know where to set the secrecy vs privacy boundary at this scenario. I do not know If am the one who made the mistake