I know that feeling. And yeah, it sucks. The best thing you can do is stay in touch with your old friends. Call, email, text, write to them. Maybe you could even visit them every now and then. If you keep the relationship up, you won't feel so empty. I know it's far and traveling prices are really high, but once you get a job and start saving money, you'll be able to visit in no time.
Don't shut other people out either. Make new friends...even if you know you're gonna leave them. Like they say, "It's better to have loved and lost than never to have loved at all" and it's true. If you shut others out and move, you'll have gained nothing. Try to think of this as a way to make new friends, and not necessarily lose the old.
You will make friends. Don't worry. You'll start school and join clubs. Things will work out. I know you're discouraged and feel like nothing will ever be the same, and you're right, it won't be the same. But I think it's better to be happy then sad. And friends bring happiness.