I sympathise, I turned 20 three weeks ago, and over a skiing holiday fell for a 15 year old - it's not as extreme as the OP perhaps, but still, I fell hard for her (it didn't help that she admitted she liked me too). I was (and am still) very much aware of the not only moral but also legal implications of liking someone so very much your junior, which thankfully stayed my hand.
To all the posters who have taken the morally superior viewpoint (I smiled when I read "snap out of it you mong"), I know where you're coming from, but to all of you who have fallen hard for someone, you must know of the anguish and pain it causes, and especially in cases where you know that you can't have her. Fancy can hit anyone, regardless of age, and it's all the more embarrassing and hurtful when you know it's socially very much frowned upon. A little more tact please. Don't judge so harshly.
To the OP: I'm afraid all I have to say is that you have to get over her, which is a lot harder than it sounds, I know. The legal consequences are terrifying if you tried anything, even if she consented and liked you. Furthermore, if she did like you, it would probably be on the whim of a teenage girl - which is what I told myself in my case - and she would forget you soon enough. It's a problem of growing up that means that older people become a lot more attached to people, whereas teenagers are emotionally fickle and volatile beings.