Dear Mr. Long Distance,
You Can't Make Her Forget
I hate to say this, but you can't MAKE her want you again and forget about the other guy.
You're Both Too Young
Let's look at the facts, first she is 17, that is very young to be looking at such a commitment, first long distance, and then marriage to boot. Heck, 21 is awfully young for it as well.
You guys are also living in two different worlds, she in high school, you in college. Different set of problems, different set of friends, different lifestyles altogether. Relationships thrive on difference, but that much, come on.
I Dreaded His Letters
Now I am not sure how much distance is involved, but let me tell you about me, real quick. When I was in boot camp I dated a guy who went in at the same time as I. We wrote love letters and it was pretty intense. He asked me to marry him and I foolishly said "yes".
Well, I stayed in Chicago for more training and he wound up in, where? Hey good guess, Florida... he wrote me constantly. I, on the other hand, slowly moved away emotionally, until I dreaded receiving this poor guy's letters. It was too hard for me to tell him it was over, so I just dropped all contact with him. Do you see where I am going with this?
She's Moved On
I really think it is time for you to move on. She already has. Treat this like a learning experience and try to gain something from it.
In the end, you can't ever change someone or make them feel something they don't. Would you honestly want to, though?