I had the same problem your going through mate, I fell in love with a girl who barely knew I was there. Whenever I went up to talk to her I would open my mouth but nothing would come out, all I succeeded in doing was making myself look pathetic. After a while it began to affect my health and I was suffering insomnia, depression, bouts of physical agony, and in the later stages it got so bad I was stuck in a wheelchair, my legs couldn't support my weight. I couldn't eat, I couldn't sleep, and all because I couldn't ask a simple question. In the end I wound up asking it in front of most of my school when I collapsed in the middle of the main quad. On an ambulance gurney, being wheeled away to the hospital I yelled out to her that I loved her. The paramedics saved me by closing the door before I could hear the laughter erupt outside.
Later that evening at the hospital I was woken up by movement in my room. I opened my eyes to see the girl of my dreams standing there. She walked over and kissed me gingerly. I looked into her eyes not sure how to take this.
She sat down on the edge of the bed and told me everything, how she had liked me from the moment she was me, and how she had always hoped when I was coming up to her that I was going to ask her out, but that whenever I just walked away she got upset that I didn't like her. We spoke at length about it, we laughed, we cried, we even swore a bit at how much our histories seemed to be similar.
So you may ask what i'm advising you to do, but I can't advise you on this, everyone is different, and it all depends on the depth of your feelings for this girl. All I know is, is that I loved my girl and right now she is standing beside me as I type this post. We have been together now for a little over 4 years and are expecting our first son in 5 months. I have gotten all I wanted in life because I took the plunge and I have never regretted that choice. Sure you may have problems, she might not like you the way you like her, she might reject you, all of these things make us not want to express our feelings but it is in our most vulnerable moments that we see who our true friends are and see how a person really feels about us. Don't hide away from your feelings, don't try to get her out of your head, we where given large hearts for a reason, we need to use them more than we currently do.