Most Helpful Girl
Guys and girls feel attraction a little different. "Chemistry" may hit, but we respond to it differently. Girls will take longer to allow it to show, though they feel it right away. Guys will try to show it right away but then back off when they feel worried that they were too hasty in picking a partner.
Her "determination" is typical of a young woman in your age range. If you don't see her as more than a friend right now, then you should be straight forward and tell her that.