Hi - Sincerely, as a woman, every time I have ever said, "Your're my best friend, that is all", I meant it. There was not enough interest/chemistry/attraction from my end, even though I genuinely liked the other person. Sometimes you enjoy the attention from a man and you keep the relationship going because you're lonely, but you know deep in your heart that he is not "the one". I would not keep pursuing her and declaring your feelings. Actually, I would take a huge step back from the relationship.
Online relationships - I did some online dating and met the most wonderful man in the world a year ago. Unfortunately, this man decided ultimately, two months after I moved my children and myself to be with him, that he was unable to devote the attention he wanted to his own 4 children while working so much to try and maintain our place too, plus child support. It is the greatest tragedy of my life, truly, because we were absolutely wonderful together. But anyway, good relationships can and do develop. Mine ended in heartache, unfortunately, but I would not write off online relationships at all. I might consider moving your attentions in another direction, though. Maybe give her some time, but often the "friend" feeling is permanent, I'm sorry to say.