She took his side instead of mine

I met a boy two summers ago and we instantly became really close. He became one of my best friends. Once I started school again him and I grew apart and stopped talking. I became really depressed around that time and was in and out of mental hospitals. I tried to re connect with him but was unsuccessful. During the time we were friends I had introduced him to my other friend Olivia. We were all in a groupchat but they would not talk outside of that. They began talking a little after I stopped talking to him and sorta became friends. Through talking she met his cousin. Then those two became close and she is constantly posting about the three of them and his cousin started dating her. This boy who was my old best friend brought old stuff up and it made everything more complicated. I kinda slipped back into my depression. She has distanced herself from me and spends a lot of time with the boy and his cousin (her new boyfriend). Whenever I’m upset about what they boy has brought up she gets mad at me and takes his side. She knows I’m upset about the situation but still continues.