I think that Keaton is talking about the fact that the woman he is singing to was the one, but then he fell out of love with her. Now he no longer truly belives in love because this woman who used to be his everything is not anymore. He feels hopeless that he will ever feel about anyone the way he felt about her, so he stays with her. There is nothing else out there for him, only her, but her as she used to be. 'The One' for him is just a memory now.
I think that Keaton is talking about the fact that the woman he is singing to was the one, but then he fell out of love with her. Now he no longer truly belives in love because this woman who used to be his everything is not anymore. He feels hopeless that he will ever feel about anyone the way he felt about her, so he stays with her. There is nothing else out there for him, only her, but her as she used to be. 'The One' for him is just a memory now.