When it comes down to 18 year olds I wont trust them more than my left ball. Im 24 now and I seen so many(one) 20 year old guys suffer badly when girl choose to change from commited girl to a community service pussy.
Anyway thats just my 2 cents and your relationship dont have to be like that. What works for your favor is - you been first serious guy in her life and she will always remember you. And as she changes her mind so often(like all teenagers) she will change her mind back to you. At some point shes gona miss you no doubt about that. Even more at some point she will need a cock and if you keep her on the good terms then...
Seriously if you learn to be ok on your own dont fall into black start drink or something she will come around in your life at some point. If you will take care of yourself you gona be over her by than. But if not then you probably gona be in shit and she wont need you.
However turn the love you have for her to yourself and live, enjoy the summer, every moment of every day. You deserve to be happy. Dont contact her let her make a move. She deserves a break and so do you - no less than that.
Doubt kills more dreams than failure ever will