Betrayal is the worst

I just found out that my ex (who I share two small kids with and is typically my biggest cheerleader in life) has been saying horrible things about me behind my back to someone I thought was my best friend and her boyfriend. When we were together, the 4 of us were extremely close and did everything together. I talk to this girl every day still. Recently we went on a trip together (the 4 of us and my kids to Disney) and on that trip is when I learned that they’ve all been bullying me saying horrible things to eachother and just pretending to be my friends. Like for 2 years. I’m fucking gutted. They’ve all been playing me like a fool. I can’t believe I didn’t see it. But honestly after reflecting, I don’t know how I could’ve even guessed this was happening. I spend so much time with my ex because we have our babies and we typically get along well and I talk to these friends every day. Every. Single. Day. Apparently they all think I’m crazy and a bad person. Even on the trip, they were texting each other while pretending to be having fun with me. I’m just confused. I don’t understand what I did to deserve all this. It’s been brutal. How do I know if I’m the crazy one??