In “Goodbye to All That,” Joan Didion writes that the “lesson” of her story is that “it is distinctly possible to remain too long at the fair.” What does she mean? How does the final section of the essay portray how she came to this understanding, her feelings about it, and the consequences of it?

In this classic essay, Didion describes moving to New York City from her native Sacramento at the age of twenty-one, and, as she freely admits, a somewhat naive twenty-one, her mind, "programmed by all the movies I had ever seen and all the songs I had ever heard about New York," with the typical feeling, at that age, that, "nothing like this...has ever happened to anyone before." Intending to stay for only six months, she falls in love with the city. As she says, "I do not mean 'love' in any colloquial way, I mean that I was in love with the city the way you love the first person who ever touches you and never love anyone quite that way again."
This initial love led her to prolong her residency, since, "I still believed in possibilities then, still had the sense, so peculiar to New York, that something extraordinary would happen any minute, any day, any month." But as the next years roll past, and the "new faces" she encounters soon become familiar and that "extraordinary" event eludes her, the city eventually loses its charm. Thus, at the age of twenty-eight, she begins to understand that it is "distinctly possible to stay too long at the Fair."
During this last period of her stay, she loses her ability to relate to other people, often tuning out of conversations. She starts to become annoyed by the signs of wealth and consciously avoids certain areas of the city, among them the most symbolic, such as Time Square and the New York Public Library. Eventually she begins to cry spontaneously, for no reason, and finds herself unable to leave her apartment, classic symptoms of the depression against which she would wage a lifelong battle. Yet, during this dark period, Didion obliquely mentions that she had also gotten married. Possibly thinking a change might help, her husband, John Gregory Dunne, decides to take a six-month leave of absence from his job, and the couple move to Los Angeles.
As Didion summarizes her decision to move, "I was very young in New York, and...at some point the golden rhythm was broken, and I am not that young anymore."

After moving from California to New York City, Joan Didion is captivated by the energy, fast pace, and excitement of her life in the city that never sleeps. She comes to embrace its loneliness and anonymity, and she relishes her newfound independence.
However, she writes that she "was not one to profit by the experiences of others" and goes on to say "that it was a very long time indeed before I stopped believing in new faces."
In making these remarks, Didion is acknowledging that she did not learn from other people's mistakes and that for many years she naively accepted what people told her at face value.
Eventually, she becomes more jaded and cynical. She grows weary of having the same conversations over and over again. She insults and hurts other people and becomes depressed.
After leaving New York City to move to Los Angeles, Didion realizes that she cannot return to New York. It has become too expensive, especially considering how much space her family would now require. She attributes her love affair with the city to her youth and states that the "golden rhythm" has been broken now that she has gotten older.

In "Goodbye to All That," Joan Didion describes moving to New York City from California—ostensibly for a few months—and how she stayed for years. As time went by, the city became less exciting and felt like more of a repetitive trap; this was reflected in her mental state and feelings of depression and anxiety.
When Didion says that "it's entirely possible to stay too long at the fair," she means that if you stay somewhere long enough—even somewhere entertaining and wonderful—it loses its sense of wonderment and becomes somewhere you don't want to be. Even if the place hasn't changed and still offers a sense of magic and wonder to newcomers, you don't feel happy there. For Didion, New York City was the fair and the only way to break the loop of disenchantment was to move back to California with her husband.
The longer Didion lived in New York City, the less capable she was of handling daily tasks. She became incapable of going to certain places. She stopped enjoying parties, because, according to her, everyone was someone she'd seen somewhere else. There were no more new faces, even when she was promised that there would be.
She says that she got married, which was a good choice but poorly timed because it didn't fix problems for her. She would cry in public places and become unable to walk down certain streets. She says she "hurt the people [she] cared about, and insulted those [she] did not." Her husband was the one who decided to take a leave of absence so that they could go live somewhere else together. By the time she wrote the essay, they'd been living in Los Angeles for three years.