Dont worry about it. Like someone said, close the book on high school. Remember her for her best, most high school relationships dont last forever.
When I was 18, I thought I knew everything. Now at 22, I realized I knew nearly NOTHING. The fun is just beginning, your life isnt even close to being over. Play the green day "time of your life" song once

, and let it go.
I'm in my 4th year of college, and I've never looked back. I barely talk to anyone from my high school. Believe it when people say you will absolutely make your best friends in college.
Considering you spent your entire high school with her, it wouldnt be right to go with anyone but her to the prom. So either go alone, or dont go at all. Don't waste your money.
Next time dont go so overboard. Don't be so pussy whipped. Realize that you will be JUST FINE without anyone but yourself. If you cant say that, then the gf isnt a solution, its an obstacle.
But the #1 advice I can give you, and you should absolutely listen to....DONT TRY TO BE FRIENDS! Its just pain. Let it be. I've seen myself and far too many of my friends torture themselves by trying to remain friends.