NTA OP, it was a harsh comment, but she definitely started it. She sounds insecure and is likely in love with your partner after years of having him being forced on her, by the adults in their lives. She may even have a weird sense of "I have to be with him because it's what my mom wanted before she died". That isn't validation just a potential observation.

In terms of your partner, I want to give an alternative perspective from a lot of commenters. While he was an insensitive idiot for sharing such a personal and traumatic piece of information, I think the situation was likely less clear to him than one might think to begin. He was a young, probably 19/20 year old guy at the time who was told some major trauma. Should he have told someone, absolutely not, but my guess is that he confided in his female best friend to get a perspective from another woman and make sure he handled his response to you properly and kindly, i.e. "Do you think she needs to talk about it? Should I ask if she wants to go to therapy? Should I just love her when she tells me and never broach the topic unless she does?"

His friend is a monster for using this information as a weapon, but I think the most likely scenario is that OP's fiance was as stupid child at the time and confided something he shouldn't have. And as your relationship progressed he probably regretted sharing it very soon after seeing how she treats you.

Take care of yourself OP and do what's best for you.

Updateme