I believe that this is an almost winless situation you've put yourself into. I understand it because I've been through it. It's kind of like digging yourself into a hole. The fact that you like this girl so much kind of means that you've dug yourself quite deep. She could save you by liking you and leaving her boyfriend for you, but that seldom happens in this situation. Even with a girl that doesn't have a boyfriend, it's quite tough. Mainly because of the friends zone that has been mentioned.
Personally for me, once I've dug myself too deep, I do not expect the girl to save me. Therefore I confess my feelings. But this is the important part. You confess mainly because you want to know what she thinks of you no matter what it might be. You confess and you anticipate that she may not want to see you anymore and she'll ignore you. You do all this, feel like shyt for a few days and you move on.
It's easier to confess and get it out of the way because it gets rid of those thoughts of hope and possibility of happiness with her. I doubt you will be capable of trying to 'change' the way you see her so that you don't have to confess to make things easier for you. Trying to change how you feel for someone all by yourself is super difficult unless you've got a vivid imagination and you can actually let this imagination run the way you treat this girl.
You could try and continue being friends and all. But if the sick feeling persists and it's spoiling your day and your mood for life, then tell her and get it over with. Just anticipate the worse. Try to even anticipate her calling you a weirdo and telling all her friends and insulting you. If you can't exactly handle that, then you've got to work on your self esteem and confidence, which is a totally different area altogether.
