How does Holden come to learn the true nature of his feelings in The Catcher in the Rye?
Holden is brought to a realization of his true feelings through his experiences alone in New York City after he leaves Pencey Prep early, ahead of the scheduled holiday vacation. Left to roam, on his own, Holden is stripped of his glib exterior, and literally brought to his knees by exhaustion, both physical and emotional, resulting in his hospitalization.
When Holden arrives in New York City, he is vulnerable and scared. He cannot go home, he does not want to face his parents. He can't stay in a nice hotel in the better part of town for fear of running into his parents or their friends. He decides to hide in a seedy hotel that is filled with undesirable people.
Holden's experiences at this hotel, including getting beat up by Maurice, the pimp/elevator operator, cause him to get closer and closer to an emotional breakdown. His escape from the hotel after his encounter with Maurice adds to his anxiety because now he is faced with the need to wander physically as well as emotionally.
Left to wander in the city, with few options, he ends up getting into more trouble and being exposed to unreliable, untrustworthy people who do not have his best interests at heart. For example, when he arranges to meet Carl Luce at the Wicker Bar, Holden is desperate to talk to someone about failing out of Pencey Prep. Instead, Carl is not interested in listening to Holden's sad story of failure. Feeling rejected, Holden instigates Luce into abandoning him at the bar, with the advice that he should see a shrink.
Sally Hayes, another supposed friend, has no interest in listening to Holden or discussing his crazy idea of running away, instead, she accuses him of being irrational and he gets mad and calls her names. She gets so insulted that she walks out on their date, leaving him rejected and alone once more.
Feeling the need to see someone who really cares about him, Holden sneaks into his apartment and has a brief visit with his sister Phoebe. His parents are out of the house at the time, and he goes through a host of emotions with Phoebe, who tells him that he will get in big trouble if their father finds out he was kicked out again. He can't stay home, even though I think that he wants to, because he is not ready to accept responsibility for his failure.
Desperate for human companionship and a sympathetic ear, he goes to Mr. Antolini's apartment only to find unstable behavior, drinking and veiled sexual advances which cause him to flee the apartment in panicked terror. By the time Holden runs from the Antolini apartment, he is so exhausted, physically and emotionally, that he cannot bear his life, in its present state, any longer.
By the time Holden meets Phoebe in the park and she comes with the suitcase saying that she will runaway with him, he realizes that it was a bad idea. What happens next Salinger leaves to the reader's imagination, but Holden gets the help he needs and is put in a mental institution for both physical and mental exhaustion.
I can think of a couple ways. First, think about the end of the book. He says, "don't ever tell anyone anything, or else you'll end up missing everybody." He's in a rest home, as the book calls it, or a mental facility of some sort. The reader is left to assume that after having a brakdown and writing the book as a way of telling his therapist what happened or led up to this breakdown. When he says you'll wind up missing everybody, maybe what he means is he actually cared about the people he thought were phony after all; otherwise, he wouldn't miss them all that much. He also affirms his belief that keeping things to himself is the best way to go, as it allows him not to relive and therefore remember things about his past that bring up memories with which he is not prepared to deal.
Also, when he cries at the carousel, watching his sister go around and around during the rain shower, it's relatively clear that this is the moment when he realizes he's "home." He found what matters to him: his sister. He cries because she's happy, and he's found a way to make her enjoy life as a little kid, even if just for a few minutes. This is just after he became upset seeing grafiti that wasn't for little kids' eyes; he is a protector, as evidenced by his desire to be the guy catching kids at the edge of a rye field before they fell off and injured themselves.