Was I Right To Break Up?

I've been with Sarah for seven years now. We moved in together early on and everything seemed perfect until recently. Last week, she invited her friend Mark over for a dinner party, but there was an undercurrent that felt off to me, it was as if they had some kind of shared secret. I couldn't shake the feeling something wasn't right between them. I waited till after Mark left to ask Sarah about it. At first, she denied everything and said he was just a friend who's been through hard times lately. But then, weeks went by without her mentioning him again, which made me even more suspicious. I started noticing how nervous she got when talking about their interactions or past meetups. One day, I found some text messages on Sarah’s phone that seemed to be coded in a way only they could understand. It was the straw that broke the camel's back for me; it felt like there was an unspoken understanding between them beyond friendship. The idea of her keeping this secret from me hurt more than anything else could have. I confronted Sarah about these texts, and she finally admitted to me that she had been seeing Mark behind my back. She told me they never touched each other physically but emotionally, there was an attraction and a mutual comfort zone that went beyond what I expected of friendship. It wasn't overtly sexual in the way most people would think, but it felt like betrayal nonetheless. I'm here now, unsure if breaking up with Sarah was the right choice or if I should have tried harder to understand her side and give us both a chance at redemption. Did I overreact? Am I being too controlling by not allowing any space for platonic intimacy with someone of the same sex? I feel like I’m losing my grip on reality, wondering what exactly constitutes cheating in this day and age. What do you guys think, am I justified in ending things or should I give us more time to figure out how to navigate this murky territory together?