I found out my boyfriend has been texting my best friend behind my back

Me and my boyfriend have been together for almost a year and me and my best friend have been close since middle school. Three days ago I found out they'd been texting each other for months without telling me. I didn't snoop, his phone was face up and a message from her came through while I was sitting right there. When I asked him about it he said they were just friends and that he didn't tell me because he knew I'd make it a big deal. My best friend said they'd been bonding over trying to do nice things for me and that I was being insecure for making it weird. Maybe nothing is going on. But they made a decision together to keep something from me for months and now they're both telling me my reaction is the problem. I don't know which part upsets me more, the texting or the fact that my best friend let it go on this long without saying anything to me. I started thinking about what was actually bothering me most. It wasn't just the texting. It was that two people I trusted completely had made a decision together that involved me without telling me and then both responded to me finding out by making me feel like I was the problem. I also thought about whether I was actually overreacting or whether being told you're overreacting by the two people who did the thing is just a really convenient way to avoid accountability. I realized that the question of whether anything romantic was going on almost didn't matter as much as the fact that they'd both chosen to keep it from me. That was its own thing regardless of what the texts actually said. And the way they both responded when I found out told me something too. Neither of them led with an apology. They both led with a reason why I shouldn't be upset. RESOLUTION Talking it through helped me get clear on what I actually needed from both of them. Not an explanation of why they were texting. An acknowledgment that keeping it from me wasn't okay regardless of what it was. We agreed the best next step was to go back to both of them separately and say exactly that. Not as an accusation but as something I needed to say clearly before I could figure out where things stood. I wasn't going in to break up or end the friendship. I was going in to stop letting them reframe my reaction as the problem when it wasn't.