Saw my ex with his girlfriend after 1 year

Me and my ex dated for 4 years. Our relationship became toxic after certain relationship. Even when he was dating this girl he kissed me and met me and persuaded me for a FWB relationship. anyways when i rejected him he made fun of my trauma anyways I thought I moved on. Bt I have been single for almost 2 years and it still feels bad that someone who had done me bad is having there time of life. He talked to my best friend and her boyfriend since he knew them when we were in relationship. He came to talk to them infront of me. telling them he doesn't think about all of those stuffs and that it has almost been an year. I cried and cried and I'm still crying idk what to do. It's breaking me inside. I thought I moved on bt today i just saw him at my school carnival. He was my first everything. We both were each other's first .