Omds I feel sorry for you bro. These situations are hard as gaining friendships and becoming part of friend groups takes timeee, and if you do one wrong thing it could severely affect that.
You should talk to someone you trust about it as I can only help you so much online, and you will know people who know about you more and care for you more and can overall help you more. The difference between people who fall mentally ill and people who don't in tough situations is they talk about it and release the negativity from their heads. Please, from all the goodness there is in the world, let it out to someone you can trust.
Also, is there a way you could break up with her amiably (as in stay friends)? Because if you do that, there is no need for people to pick sides. However, I totally understand if it's not that simple, as most times it is not.
Is there anything you enjoy, like hobbies? Because reintroducing these stuff really brings people back to being themselves as you may not have as much fun anymore, and need to have that break. Enjoy yourself! Also, the thing that will make you feel worse is thinking it's worse. Your mind being negative makes everything that is bad even more bad, so don't deepen the situation and think optimistically!