First off, try and think about what it is that this girl means to you. WHY is it that she means that much to you. You say you're afraid to talk to her, have you? You should!!
Sometimes we see people and become infatuated with them, like they can somehow add meaning to our lives, or add something that we don't have. That's not a fair thing to do for the most part. No one person can complete you, it's not fair to them to put them in such high regard because then if they somehow dissapoint you (And they will, everyone does.) then you are right back where you started and are crushed all over again.
I know how you feel, I've been there. Try and do other things and fill yourself up with other things. It's not healthy to obsess over a girl. If you find other interests, and do other things and try not to think about her, chances are you'll have more confidence and you WILL be able to go talk to her because your life won;t be consumed by her so much, she'll just be another girl that you like.
Also, you mentioned you're a senior, congratulations!! Do you have your colleges picked out, are you going to college?
Im a little older, and am through school, but TRUST ME, if you DO go to college, you'll meet a ton of girls, I know.
I met my wife in college, and am glad I did because of where I was at emotionally and maturity wise.
Don't give into the depression. If it's REALLY bad, go seek professional help and if need be, depending on your doctor, get on some antidpressants. Although be careful with those, people between 14 and 25 or so I hear have had some issues with anti depressants COMPOUNDING the depression.
Try some exercise also! That has helped me immensely. It raises your dopamine levels naturally, and can become just as much of an addiction as people.
Keep your head up, it'll be ok. I know it hurts, but try and find things that can kind of distract you from it and it'll eventually fade away. In the mean time, it doesn't hurt to talk to her, befriend her, who knows; you might come to know her and find out that you really don't have the same feelings you once did.