I know you are hurting but you have to try to put your feelings aside long enough to just look at the facts okay?
She told you from the beginning she just wanted to have fun.
You were her summer fun.
And that's not such a bad thing! At least you ended on a positive note and she still considers you a friend.
Your problem is all the stuff you are now telling yourself to build this "romance" up into the "love of your life" type when it just wasn't.
What this was was your first relationship.
And while that can be pretty special and something you might never forget, it ran its course.
You are very young and very inexperienced so please pay attention to all I am telling you because you need to get this stuff.
You did not do anything wrong!
And you have to stop ruminating on all this and kicking yourself for your imagined failings.
Take what you learned from this experience and build on it
There will be other girls, IF you allow yourself to go after them.
DO NOT make the mistake of deciding she was your "one and only" and then using that
excuse to keep you from having to go out and meet new girls.
You are making a choice to be depressed about this with the story you are writing in your head.
Take control of your emotions, feel them, but no not let them rule you!
It is hard to forget someone special, but the best way to do it is to get real busy.
No more sitting around and mooning over her.
Every time she pops into your head you have to consciously make yourself think of something else.
You will have more luck doing that if you have other things going on to occupy your time.
This is the kind of stuff that separates the men from the boys. Time to grow up and join the rest of the adults around you