My (31M) fiancee (30F) and I have had a pretty steady friend group for about 10 years now. One of those friends is Derek (31M).
Our friend group has encountered some "drama" over the years and almost all of this relates to Derek in some way. Derek has dated two of the other women in our friend group and both relationships ended poorly. After these relationships ended Derek had the expectation that we would no longer invite his exes to outings that he was attending and, while he never came right out and said it, seemed perturbed that we continued being friends with them at all after their respective breakups. Derek deals with some pretty serious mental health issues and is quite unstable which unfortunately has contributed to a lot of the drama over the years.
About a year ago, my fiancee and Derek were hanging out together alone. This was not out of the norm for them. However, when my fiancee returned home, she told me that Derek had told her he was upset with me about something I had said when we hung out last. This rubbed me the wrong way as I didn't appreciate that Derek would talk to her about this "behind my back" rather than just come to me and talk about it. I chalked this up to Derek trying to start his usual drama decided not to say anything to him about this and instead waited for him to come to me about this, which he never did.
I then went through a period I would describe as one of the most difficult of my life, for various reasons I won't get into for privacy's sake, but suffice to say a lot of really difficult and painful events happened in quick succession. At this time I felt my fiancee was not as supportive as she could have been. In fact, she seemed a lot more concerned with the fact that I hadn't "made things right" with Derek. At this point I hadn't talked to Derek at all since he spoke to my fiancee, and with everything that I had going on, this was at the absolute bottom of my priority list.
About a month later it was my fiancee's birthday. We had a lovely day together, and I asked her a few times throughout the day what she'd like to do for dinner and she seemed evasive. Finally, right at dinner time, she admitted she had plans with Derek and I was not invited because Derek was upset with me still. I told her I was uncomfortable with her going out for dinner on her birthday alone with Derek, even more so that I was specifically barred from also attending. She went anyway.
This caused quite a bit of turmoil for us as I felt disrespected and frankly questioned if there was more to her relationship with Derek. My fiancee apologized for not being honest with me about her dinner plans and I asked that she not spend time alone with Derek in the future. She agreed. However, around this same time, Derek was essentially having a falling out with everyone else in the friend group aside from my fiancee. My fiancee argued that, since Derek was now on the outs with all of our friends, she would have no choice but to hang out with him alone. I maintained it would make me uncomfortable and asked her not to.
For the next several months my fiancee told me that Derek had stopped communicating with her. I would periodically ask if she had heard anything from him and she would always tell me no, nothing at all.
Well, a few days ago I was brushing my teeth while she was in the shower and she had left her phone face up on the bathroom counter. Her phone lit up with a text from Derek. It was immediately obvious from the text that the two of them had plans that day after she got off work. She had told me that she was going be working late that day, but I now realized this was a cover for seeing Derek. I immediately confronted her and she folded and told me they had been in contact the entire time, even when she swore up and down that she'd heard nothing from him.
She swears there's nothing going on between the two of them, but honestly I don't know what to think. I don't trust Derek at all anymore, and I can't contact him at all because he's had me blocked on all platforms for months. I feel like the trust in my relationship has been broken and I don't know what to do to fix this. I just need some advice on how to handle this.