I think you've already gotten excellent advice. There's little I can add that isn't already said. I will say, guess how many of my high school friends I still would consider close in my life? None. Then guess how many of my college friends I would consider close in my life? None. And, I went on to start a career and made new friends. Life is full of phases, like it or not, ready or not, and each one offers a chance to start anew in some way.
If I knew how little things at your age (other than education) really mattered, I would have relaxed a hell of lot more than I did, and enjoy it. I don't mean that to sound condescending. I was told similar things when I was your age, and the advice seemed cheap to me then because everything felt immensely important in the context of that moment. But, age will give you a certain perspective, and with that a certain nostalgia. And, the things you'll come to regret is the times you let yourself be overwhelmed by things that were not that important in the long and grand scheme of things, which caused you to miss an opportunity to experience something.
One of my Facebook friends lost her 3 year old son to a rare childhood cancer a couple years back. Just a bad roll of the dice, something went wrong in his body, and he's gone. Died in her arms one night. Every single day she posts something about missing him. A friend of mine from high school just lost her husband to a 2 year battle with a brain tumor. They have 8 kids, and she was a stay at home mom and has no job as he was the breadwinner. I have photos on my timeline of her saying goodbye to him at the hospital as their 3 year old boy watches from the nearby hospital bed. He probably won't remember his daddy. Three of their kids were his from a previous marriage, and the family sued for custody and has taken them back from her breaking up her family. A doctor once told me "Don't sweat the small stuff. Everything outside the body is small stuff." There's a lot of truth to that.
Salvage the relationship with that girl, she sounds like she's after what's best for you, and just go a day at a time from there. Before you know it, something else will present itself and you'll be off on another one of those phases I talked about. It may seem like the end of the world right now, but one day you'll look back on this and see it's just one page in a much larger book. You just need to write a few more chapters to see it.
Good luck, and chin up. You're going to be fine come what may.