She thought everyone else saw the ghosts, and that they all worked with them as a fundamental part of the world on a daily basis. But, she couldn't see the ghosts. She believed that there was something wrong and different about her that she couldn't see them.
So she learned to make inferences about the ghosts and to behave as if they were there, but she was intimidated and frustrated by this mysterious problem that no one else seemed to have. She kept working on it, though, and year after year, she was sure she was getting closer to actually seeing the ghosts. She thought that one day, she would see them and then everything would finally make sense. She devoted so much energy to this project that the necessity of interfacing with the real people and things around her was usually trivialized and neglected. She didn't think she was capable of doing it well, anyway, until she had fixed her problem.
Then, one day, she realized there was never any such thing as ghosts at all.
Of course, her habitual ways of thinking hadn't changed just because of one new realization. So after that, she would frequently find herself falling into the same types of behavior and making the same assumptions as before. But they all seemed to be based on superstition, now. Every time she noticed this, she wanted to laugh. Or at least, she wanted to want to laugh.