Search Results: tell a girl you barely know that she is pretty

I know I've screwed this up in every possible way so just tell me why I'm such an idiot.

There's this girl that I've 'known' for over 1.5 years and had one class with for a year. For the first half of the year we didn't even talk, let alone say hi. We have one mutual friend that has classes with either of us and one day he tells me this girl mentioned me in a convo, what was to my surprise because I didn't even remember her, and another day when we all three happened to be together she called me by my name which was a surprise too because I don't usually go remembering names of the people that I've barely had any contact with. So after that time we had some conversations, mostly in groups where I got to know her a little better and found out we have a lot in common (say out personalities, views etc. are pretty similar but not going into details). She's also like the cutest person I have ever seen, her smile alone makes my day better when I see it across the hall. So here's where my imbecility steps in... I'm a very shy person and I'm only comfortable around my friends when there's no strangers around, so if there's someone that I like around me I'd rather blend in the wall than have them talk to me... So there have been some situations when we'd have classes next to each other and this girl would come up to me and try to strike up a conversation (I hadn't noticed her trying to talk with any other guys from our class that we had together), and I instead of being nice and actually talking to her ended it with one liners like 'yes/no' that put an awkward silence for the next 10 minutes till friends of either one of us came and we parted ways. We also look at each other quite often, although I break it as soon as she notices me. I've noticed she may be looking in the opposite direction just to fastly turn around and of all the people around look me in the eyes... I don't know if any of these is a sign that she may like me but I've started to believe that she may actually think I'm some kind of a creep that keeps looking at her and breaks the eye contact as soon as she notices it lol. I also found out she's had a boyfriend for a while already (2 years or so) so was she just trying to be friendly all this time and I took it the wrong way? Because if I know that she's got a boyfriend then it's like 99% unlikely she'd be interested in me anyway. Quite an essay lol, anyone that is a bigger idiot than me here?

short version: there's this girl that I like and that may (or may not, more likely) like me too but every time she tried to talk/look at me I ignore her and act like a mean idiot. Why? :|