Thank you for writing to Alterheros. By what you're telling me, it sounds like you and your friend of 6 years often hang out and know each other really well. You seem to be a great friend to her. It's easy to tell because you sound like you care about her and your friendship very much and you think from her perspective, so she wouldn't get hurt in any way. If you've known each other for six years, which is a long time to develop a strong friendship, then I would assume she is just as good of a friend to you as you are to her. If you are honest with a true friend about your feelings for her, she should still care for you the same amount as before, even if she doesn't return those same feelings. So at this point, I think honesty is likely the most straightforward method to let her know how you feel. Bottling up feelings for too long is unhealthy for our bodies and our minds. So it's better to let it out.
Have you tried to talk to your friend about this and observed how she reacted? You don't necessarily have to tell her all at once how you feel if you don't feel comfortable, but you probably have these feelings buried inside you for awhile and you'll eventually want to let her know. By giving hints here and there, you can probably get the gist of what she's feeling. I know it takes a lot of courage to tell your friend how you feel about her, especially if your friendship is at stake. But maybe she has the same feelings for you deep inside, and you will never know if you don't find out. So explore the possibilities because you are only 17 and have plenty of time to let her know.
However, if you're still hesitant about talking to your friend directly, or you feel really uncomfortable, or you're just not ready to talk to her yet, there are always counselors, teachers, or friends/family members that you are really close to that you can talk to first. These people can offer more help than you can possibly imagine and you don't have to talk about anything you are not comfortable with to them. I hope this helps, and feel free to ask us others questions you might have.
Zhen for Alterheros