You sound just like I did about ten years ago. I have done the exact same thing, and had the exact same result. The truth is, once you reveal intentions to a girl in that way, it's really up to her how to respond. She probably had plenty of time to think this over and she has made her decision. If she decided that she wanted to date you, the two of you would be dating right now.
When a girl discovers a guy has interest in her as more than a friend, she will either do one of two things. She will open herself up to him, or she will give him the cold shoulder. The truth is, and I have heard this from girls personally, is that they back off from being friends with him. They basically worry that they will have to censor themselves around you, back off from any nice gestures they normally make with their friends, for fear of leading you on and generally worry that they are going to hurt you.
So what to do about this situation? There really isn't much you can do at this point. Is there any kind of magic social maneuver you can make to change the way she feels? Nope. There is no poem you can write, no letter you can carefully compose, no prayer you can say, no bouquet of "friend flowers" you can give her. Sadly, you kinda blew it.
But don't get too down on yourself. It was worth a try, and courtship and dating situations are all about trial and error. Very few people get it right the first time, and many times it is years of trying before they do get it right. Sometimes when they do, its purely by accident.
My advice in the future is not to try to make a girlfriend out of a friend. Never look at being friends with a girl as a stepping stone to having her as a girlfriend. Once she has decided you are not a dating prospect, which tends to be decided on within the first five minutes of meeting you, you get filed under the friend zone. So what about all these examples you see on TV, where the guy and girl are best friends and end up as a couple? There's a reason why those are on TV. Not because its a reflection of the way things are in real life, but because those are the exceptions rather than the norm.
And that's the best advice I can give.