My best friend is dating my ex… and expects me to be okay with it

I don’t even know how to process this properly. My best friend — the person who knew everything about my relationship — is now dating my ex. They didn’t tell me right away. I found out indirectly, and then they confirmed it like it wasn’t a big deal. Their explanation was basically, “we didn’t plan this, it just happened.” And now they expect me to be mature about it. To “understand.” To “be happy for them.” To not make things awkward. I’m trying to figure out if I’m being unreasonable, but honestly… it hurts. Not even just because of my ex. But because this was my best friend. The one who knew how hard that breakup was for me. The one I trusted with all of it. And now I feel like I’ve lost both of them at the same time. I don’t even know how to act around them anymore. If I react, I look like the problem. If I stay quiet, I feel like I’m disrespecting myself. I’m stuck between wanting to walk away completely… and not wanting to lose someone who was important to me. Has anyone dealt with something like this? Did you cut them off… or try to accept it somehow?