Well my friend, you and about a billion other guys are in the same spot. I'll say this, women at such a young age (believe me I know, I'm 18 too) are just too immature to even realize what they are passing up. Look, she considers you a friend, because you're so nice to her. You're like, putty in her hands. She has a problem, you're there with a warm cozy shoulder prepared to let her cry on it.
I'm sorry, but it just won't work, not to get the girl. Be kind and supportive after. Girls want to be chased... you're just, too easy right now. They want a challenge, at that age. That's why you always see these players, who are quite rude, getting the girls.
Now if you're in the friend zone, the odds are you won't be able to get out fast. What you should do, is be more alouf. Note, don't be mean, just don't be "all there". Don't always agree, don't always be as sweet as honey. It won't attract her in the way you so desire.
In fact, as crazy as it might seem, your best bet would be to try and find a different girlfriend. If you don't invest too much emotion into any given girl, and are confident, then she'll become more attracted to you. If you're dating someone else, it shows that you have your own life; something most women seem to like.
I suppose these instincts come from primitive ages past, but they still are apparent in women. Not to be demeaning, and of course there are exceptions, but it seems to be true so far as I've seen.
So to sum it up, be more casual, try not to care so much, be confident, and when I said confident I mean; comfortable and at ease with being yourself even in public situations. Girls tend to be able to tell when you're faking it.
Good luck!