Speaking from a male perspective im not sure how much this will effect you but im going to speak anyway.
My girlfriend recently dumped me because she said she now saw me as more of a friend (yeah...one of those dumpings) than as someone she loved.She basically said what you said to the 21 year old. you cant force a feeling. She said there was a point when she did love me for the first couple of months but then she said i was more like a really good friend she didnt want to loose. I was so hurt when she said she didnt want to be with me because everyday of the 7 months we'd been going out i had said i loved her. And i still really did...well...actually i still really do. present tense.
This is probally what that 21 year old is thinking now.
That might seem mean to say that but if you wait a sec im going to go from the position of the 17 year old.
Now that who girl dumped me; (like you dumped the 17 year old to begin with) hurt me bad, but if she came back to me now, 2 months later and said "would you go back out with me", i still love her so much that i'd be straight back in there. no matter what shes done to me or how much ive been hurting, I still love her so much that i would take her back with no questions asked.
Now, as i said at the beggining it may be different for you because the people your talking about are girls, and im a guy. But if that 17 year old still loves you and REALLY loves you then she wont care what youve done, she'll just be happy to have you back, just as i would be happy to have my girlfriend back despite her dumping me.
So my advice i guess, would be.
If you think this 17 year old girl is amazing, kind, considerate, wouldnt hurt a fly, is understanding and forgiving, then ask her back out and if she really is the person you think, shell just be glad to have you again.
If not...then she isnt the person you thought she was.
Good luck.
Hope you get what your heart desires.
And in future...hold on to the good things you got. Because you sometimes dont get another chance with them. i know i probally wont.
(now go listen to Bon Jovi - Living on a prayer, and pay special attention to the lyrics)