I'm so conflicted by this revelation and it's keeping me up all night, as I can honestly say it's my first time having a crush on a girl, nevermind the girl who's been my closest friend for almost 10 years now. I don't want to tell her because I don't want to make our friendship awkward (which it definitely will be, especially if word gets out to the rest of our friend group), but at the same time a part of me wants her to know? Ughhhh crushes are terrible things.
Hey, I feel you. I know it's hard crushing on someone you have a cherished friendship with, but if I can give you any advice... Why don't you try and talk to her? I mean, go slowly first; try to get her opinion on what she thinks about this kind of relationship first, but do it subtly. Like, approach the subject (even by joking) to see how she reacts. By joking, I mean something like "I like you so much for supporting me I could just marry you!", things of the sort. If she reacts badly you can just say "nah, I'm just joking, I'm happy to have a supportive friend like you", see. So if the plan backfires, you have something to cover your real intentions.
Depending on how she reacts, after a while, maybe you can even talk honestly to her what you've been feeling. Even if she doesn't react well, I still think you should be honest with her, because... you know, there's no telling what can happen. What if things work out well? I say this from personal experience, I got a crush on a friend of mine and was afraid of telling her because I didn't even know if she liked girls, that could spoil our friendship, she could avoid me, etctera... I tried to ignore it but like you said, it was all I could think of, and it was getting me sleepless. One day I just got the courage to tell her everything I felt, and guess what? We're madly in love until today and she's the best girlfriend I EVER HAD, even being the broken person I know I am. She's actually getting me to like myself a little bit, it's like she's putting my broken pieces together, you know? I can't even imagine me without her, just like the day I decided to confess I couldn't ever imagine she'd return my feelings.
You never know what may happen, and sometimes all you have to do is take a deep breath and take the first step. I'm sorry for saying all of this openly (I tried sending you a PM but I didn't know how), but I wanted so badly to tell you this that I couldn't help it. And maybe my advice is useful for somebody else too... :')