Thats the big problem with high school relationships, graduation usually means the beginning of the end. Enjoy yourself while you can, breaking up really does suck but its a better use of your time now then not doing anything.

PS, there is a horrible truth of life that there are women who like to be controlled. They wont say it or may not be aware of it consciously, but they will always go back to the same or another controlling man. If this is your case there is not much you can do about it, just show her the best cooperative relationship and maybe she'll have a change of heart.