Well your like me in the sense, you'll never actually put the thoughts into actions.
I've been in the position you've been in, but with a girl ALOT further away from me, and not knowing, etc. Never found out, and never will probably.
Don't miss a chance is all I can say
I know your worried about losing her if you tell her, but what your feeling is a natural thing towards her. You can say it in such a way that if she says no, you'll most likely carry on as normal. If she says yes, you have a chance!
(Heck, I should use my own advice like I've been saying, but yeah, I know how hard it is mate)