I think that Whistler's words could be interpretted in many ways.
First, theres the whole Buffy only has herself thing. Which makes since as we see Angel almost getting Buffy, but she states that she has herself(as nerd4hire pointed out), but thats what its always been like for her. I mean shes the Slayer. Even with friends and family and a semi-life, Buffy has always been alone. She may walk in the world during daylight, but she lives at night. Its her destiny, its what shes supposed to do.
Second, it could mean Dawn. Dawn would come about 3 years from now. We've never seen Buffy love something as much as Dawn. Although I think thats probably not it, but it struck a cord to me.
Third(which I think to be true) is her future, not who she is right now. In Becoming, Buffy was at one of her hardest times in her life. She had so much to live for and so much to do in life, but it was all becoming a burden(no pun intended), but she had the potential back then to change the world and make her life better. I think thats one of the reasons Buffy always worked so hard: so she could be normal one day. Its what she deserves.