Be careful on how much you share with others
This is raw advice so apologize if it's slightly nonsensical, I think this might just be my way of processing.
I was in a talking stage with a potential and things were going great. We had amazing chemistry and could talk about anything from the most serious to the most random things for arbitrarily long. Our worldviews aligned in major ways, and we found ways to compromise on the little stuff. We had minor arguments sometimes, but always found a way to resolve things amicably. Our families, from parents to siblings, got along great. I had given up hope on finding a partner years ago, and she reignited a hope in me that I hadn't felt in years. My family used to comment on how happy, full of life, and optimistic I seemed.
A week before our engagement, I got a text from a random person. He claimed that he and this girl had a history and was able to rattle off very recent personal details about my life, some accurate, and some not, as a way of proof. He was able to provide enough details about hers (historically) and provided pictures to poison the well. I think we both handled the aftermath poorly in retrospect, I went into information gathering mode and she shut down, which to me seemed like an admittance of guilt. The families were given a rundown of what happened and things naturally ended.
We did eventually reconcile just for closure. Her side of the story is that the man was a former talking stage whom she had rejected. He is now dating her best friend. That "friend" had been feeding him every private detail she shared about our relationship. Out of jealousy and a desire to burn her prospects, he used that inside information to craft a narrative that looked like she was cheating.
Every emotional part of me wants to believe her, but my rational side is wary of being naive. Regardless of the truth, the bridge is burned. When I tried to see if our families could reconcile, it became clear that egos were too bruised. My family can't move past the suspicion; her family is insulted by the way they were questioned. Deep down, I know they're right because once any level of suspicion has taken hold, there's no saying when it will come back to haunt us in our marriage.
I'm choosing to believe she was a good person that didn't betray me, and that this is instead the qadr of Allah and a painful answer to our istikharas. If her version of events is true however, the final nail in our coffin became her oversharing with her friends, and my involving my family with details before giving enough time for potential reconciliation.
I've had breakups before, but this is the most painful experience I've yet to encounter in my life.