Hey man i know exactly what you mean i've been through that. I going through the same thing right now but this girl knows i exist but when i'm right by her she sometimes doesn't even acknowledge it. Anyway my friends give me tons of advice and the one that I think might help is don't regret it because regretting sucks big time take my word for it. You don't want to wonder what if in the future. Just give her a smile when you walk bye her, and if you know her try to become a better much closer friend that she feels comfortable talking to and being around. See i'm a Sophmore in highschool and theres this senior that used to like me, i think, and she used to try and talk to me and never tried talking back and i shes not into me anymore i think and know i regret it so what im trying to say is just don't regret not trying because that sucks even more.
P.S. Good Luck and try to get to know this girl.
LOOK AT MY POST "I NEED ADVICE DESPERATELY" its still on the home page list