OP finds out his fiancee's past

Hey guys I hope you can help. Please believe me I do love this woman so much but I feel like I'm sacrificing my own mental health to keep her around. It's gotten to the point where I'm starting to think hateful things about her now and that's not fair to her. But I can't help these thoughts rushing through my head. Over the 8 years we've been together she's never mentioned anything like this, I feel like I don't even know who she is now.

I found out about this because we saw one of her old 'friends' from a while back while we were out a few months ago. Him and his girlfriend came and sat with us after he recognized my fiancee. He pretty much spilled all the beans, just by asking questions about how her life is going. Asking if me and her were open, if I was bisexual, if we share, etc. He and his girlfriend are and his tone of voice and general gist of the conversation implied that my fiancee had this type of relationship with him too and her giving him dirty looks and a stop talking now stare basically confirmed that. She apologized a lot because she could see that it made me uncomfortable but I didn't really want to hear what she had to say about him and the situation so I just told her it was OK and not to think about it.

But I've decided that I'm not cool with this. The anxiety and lack of trust I have for her after this are just too much to handle. I'm not coping and am becoming in a way, depressed. I can't help but think she would have tryed on this kind of thing later once she had more leverage over my life post-marriage. I'm genuinely scared at the moment because her family seems to love me and she happens to be the only woman I've been with who my mother liked and she'll probably never let me hear the end of it too. I was coasting along just trying to ignore the whole thing but it's not working. How do you even break up with a person who's life is basically sewed into yours? I don't know how people with kids manage to do this shit. I need help here.