Haha oh.. Well i'm 6ft and i'm female.. I don't know how her mind works, but if you're her height now or almost there, it shouldn't matter. Plus most girls know guys finish growing later than we do.
If you were like 5'6" or 5'7" it might be awkward, but at 5'11 1/2" or 6' you should be good to go as you are :-p
anywho, if she's that superficial that you wouldn't have a chance unless you were like 6'3", you don't want a girl like that anyways.
Good luck! Teenage girls are a pain in the butt :-p I should know, I used to be one haha