Caught My Partner in a Taboo Fantasy Roleplay with Another Woman

I recently ended things with my long-term boyfriend Alex because I caught him engaging in what I consider a major breach of trust and our sexual boundaries. We've been together for five years, and the foundation of our relationship has always been honesty and mutual respect for each other's needs. About two weeks ago, I was going through his phone and found a series of texts with another woman named Sarah about them playing out a BDSM roleplay scenario he had mentioned wanting to explore before but we hadn't tried yet. He never told me it was happening in reality. In one particularly detailed message chain, they were arranging a time where Sarah would come over dressed as his boss and use her authority to make him do things he wouldn't normally consent to out of fear or pressure from an authoritarian figure. Alex had expressed interest in this kind of roleplay but never mentioned pursuing it with anyone else behind my back. What made me even more upset was when I later found explicit photos on his phone of him and Sarah together, clearly engaged in a session of the roleplay he told me he would only ever do with me. He admitted that they had an actual meeting like this one night after work without telling me. The photos showed him on his knees in front of her while she was wearing high heels, giving orders to him verbally and physically, all while I thought we were still working through our kink exploration together. When I confronted Alex about it, he tried to brush it off by saying that it was just a roleplay fantasy with no real meaning beyond playing pretend. But the level of detail in those texts and photos clearly showed genuine engagement from both parties on an emotional level, not just physical. And even though she’s just another woman in his life outside our relationship, I felt so betrayed because of how much this kind of dynamic has meant to me as we've built intimacy. The sex that night after finding all these things was rough and angry on my part. We both wanted it to be about resolving the tension between us, but it ended up feeling like a release for him while I couldn't stop thinking about what had just transpired. It's going to take some time before I can trust again or even consider reconciling given how much this has shaken me.