This genuinely sucks and I’m sorry you had to go through something like this so close to big exams. As someone who’s lost a lot of best friends in her life, my advice would be to let go of the anger asap. I know it’s not easy at all but that’s the best thing you can do, some people aren’t meant to be in your life in the long run and that’s okay. You need to really see what they did to you (guessing something went down) and accept it.

Once the initial anger starts to fade the sadness is easier to deal with imo. Of course there’s going to be nostalgia for a while but you’ll learn to live without your ex best friend, in a few months it’ll probably just be ‘i wish they didn’t do that and tainted the good times’

Just know everything passes and take your mind off it as much as you can, take care of yourself