Hi, and welcome to PC!
You are suffering while she is playing...period! Her feelings aren't as serious as yours and you are suffering while she's playilng you! I say go on and enjoy your summer, dismiss thoughts of this girl and have a good time yourself, including dating others.
I'm old enough to be your grandmother, but I did much the same to a fella in college, who pined for me as I played around, and gave him the runaround. And he remained faithful and was ALWAYS there for me! He should have walked away at the first sign of my waywardness, but he didn't! I regret that I treated someone like this, but it is valuable expreience in relating what you are going thru. Just the fact that she went to the prom with this fella should be enough for you to say..."Bye!"
Patty