WIBTAH for questioning my engagement after I found out about my fiancée's past?

I (41M) have been with my fiancée (33F) for 6 years and we just got engaged this year. The relationship is great in most ways, but the sex could be better - she's a bit prudish. Sex with the lights off, only one or 2 positions and she doesn't like oral (giving, likes receiving) - that sort of thing. But I put up with it because she's great and I always felt like I was punching up?

She's always been very cagey about her past and doesn't talk much about her family back home - she moved here shortly before I met her - apparently, there was some sort of disagreement or falling out. But there was a death in the family this year and she went back for the funeral and they reconnected and they invited us to go and stay this Christmas. So we went this past weekend and I met them for the first time and at first, all good.

We were at this family party/gathering and an older cousin (48F) of hers pops over and my fiancée got really awkward. She's mentioned her before that she never got on and tolerated her but is a bit of a narcissist. My fiancée went away to help her mum with something and the cousin just kept talking to me. I could tell she had a bit to drink and was a bit leery and said something like how does her idiot cousin get such a great guy like me when she was such a slag in the past. I was a bit dumbfounded by that and was about to say something when fiancée pops back really angry telling her to fuck off but the cousin wasn't shutting up and started giving it back to her. She started saying how my fiancé was a real slag as a "kid" and had so many abortions before she was even 16.

My fiancée just went in herself and clammed up, I tried to take her away and tell the cousin to fuck off but she kept following us and then cornered us saying she then became a prostitute to support herself at uni and even was a porn star. Apparently, someone leaked a porno of her doing a gangbang to the family and so many of them fell out with and disowned her and that's the reason apparently for the falling out. By this point, her dad could see what was going on and laid into the cousin saying she needs to go and she did. But my fiancée was in tears at this point and we left and got a hotel.

In the morning, we had a conversation about what she said and apparently it's all true. She was sexually assaulted by a boy in her year at school at 14 and it made her really promiscuous. Apparently, she was well known for being easy to have sex with and did all manner of stuff and had 3 abortions by 16 (not as much as the cousin made out). She said it was true about the sex work, she became an escort and did porn throughout uni. All sorts - lesbian, gangbangs, hardcore stuff - things I can't even imagine. That's why she's so prudish now, she's so ashamed of her past. She genuinely doesn't know how many people she's been with but it's easily in the high triple figures.

I just couldn't say anything. I was stunned. She said if I knew, I'd probably not get with her and I didn't really disagree with her. It turned into an argument and some hurtful stuff was said on both sides and I just walked out. I actually got the train home yesterday and I'm here now - I'm going to spend the day with my parents.

Fiancée has been messaging and trying to call me non stop and I told her I just want time to think. She finally understood and has stopped messaging me now, I'm going to try and think and we agreed to talk when she gets back after Christmas.

I genuinely don't know where to go from here though. On one hand I have this amazing woman who I wanted to marry and spend my life with. On the other, I can't look at her the same now. Don't get me wrong, I can't blame her for the past especially the SA (as a previous survivor myself I can especially relate there) but I can't get over what she said she did and especially the sort of things she did she doesn't even contemplate now with me. Loads of things are going around in my head.

Am I an asshole/overreacting/wrong etc if I ask to hold off on the engagement until we've worked on this?