Please help

Basically, I have (had) thia best friend. We were insperable, we hung out all the time after school, on the weekends, during the school holidays and we even went on holiday together. We loved each other in an 100% platonic way. Then I made this friend in my science class, she was nice and we got on really well, but I never hung out with her as it would upset my best friend. Then my best friend and my science class friend started talking. I didn't mind. We're all girls and my best friend and my science class friend were both lesbians. They started dating. Again, I didnt mind. It was a good healthy relationship until September this year. My science class friend moved into my form class, the one that I had with my best friend. They stared to alienate me from all conversations, even if I was next to them. They'd make plans in front of me and ignore me. They would hang out at the weekend and after school and in the school holidays. I'd try to see my best friend, I'd ask her if she wanted to do something but she'd tell me that she had homework or revision to do. But later on id see her hanging out with science class girl on social media. My best friend got rid of all of our traditions to make her girlfriend more comfortable. My best friend ruined our 4 year relationship for her girlfriend. And she thinks that its okay. I'm hurt and angry. She won't listen to me when I explain how much she's hurting me. I already have medically diagnosed mental health issues and this is making them worse. I can't go into school because I get alienated by them and I'm so sad. Please tell me what to do