I'm not a lesbian but WHY do I get so nervous around my friend?

[why]

She's extremely gorgeous and I like to stare at her a lot lmao. We get along great have a lot in common. But I feel like I can only talk to her normally over facebook and texting, Twitter...not in person...in person I get so nervous and I don't even understand why...

Before every class I have with her I have to prepare myself mentally...And when I think about seeing her my heart beats so fast, I feel sick and anxious. I start shaking...

She always smiles and stares at me, says hi to me...she used to be even friendlier, invited me to do things with her (I'd decline sometimes because I got nervous) we always sat together in class, she'd hug me whenever she saw me...but then I started distancing myself from her, ignoring her a little...

She probably wonders why I'm so detached and ignore her so much. In class I hardly even look at her and she just stares at me but I don't know what's wrong with me! I get so nervous around her its exhausting and it makes me act really awkward around her. And it sucks because I really want to be her friend...

I think I get so nervous around her because I feel inferior to her, not good enough? I do have really low self esteem. But I noticed on days I feel pretty, I'm relaxed around her and we talk just fine but that's really rare that I.feel pretty. That's only happened twice out of the 3 years I've known her