How to impress a 15 year old school girl?

I m a 18 year old boy in college and I met a 15 year old girl in my guitar cles which took place for about a month.I slowly began loving her but could not say anything to her about it cause of our age differences and bcoz she is a very descent and simple girl. After the guitar cles got over i tried to forget her but instead i miss her more than ever..Recently after more than 6 months ped since we met , out of desperation, I send her a birthday card through a third person who is a girl and at her school.She couldn't recognise me probably she forgot my name and whereabouts..even her whole cl came to know that she received a bday card from some guy and later she told her parents that an unknown person is sending her bday card..My whole image has got spoiled in her eyes...I m really depressed and want your suggestions wht shud I do???