In the second half of "The Catcher in the Rye", Holden Caulfield's mental situation becomes all the more clear to the reader as being seriously detrimental to his personality and powerful over his actions.
One example of this is when Holden is discussing how he got an F in Oral Expression as a result of his digressions. This immediately may not seem as though it supports the point that he is becoming sicker and sicker mentally. However, if his digression is viewed as something that Holden still possesses looking back on the class from his current vantage point, then the reader can determine that his digression is getting progressively worse. His worsening digressions are a result of his mental inability to focus on one topic and exhaust it before moving on. He also appears to be unable to stay on one discussion because of his advanced depression and the unpredictable nature of it. His thoughts about a perfectly happy event could be misconstrued by his own mind so that the happy thoughts become those of extreme emptiness.
Another part of the book that can wrap some of the reason's behind Holden's problems into a neat little package would be when he is visiting the Museum of Natural History. He looks at the glass cases and the animals and sees that they are the same as they always have been, and Holden is comforted by this, and yet saddened.
"Nobody'd be different. The only thing that would be different would be you. Not that you'd be so much older or anything. It wouldn't be that exactly. You'd just be different." pg 121
In my own life, I have things that are wonderful that change and things that I can count on to stay the same. I measure my change by my repeated returns to the things I love and can be comforted by. With Holden, I believe that his feelings about the museum are wistful remembrances of when he was a child and a dependable thing, as silly as a display always being there, could ground him and keep him from straying from his path to adulthood. As a young adult, Holden still needs the organization and unfaltering reliability of the museum. It is sad that as an adult, it takes more than a museum to keep Holden from falling, while once it could heal him.
Holden's instabilities have come out and in the end, a forlorn Holden is telling the story from an institution of sorts. There is hope of a new start for Holden but sometimes hope is not a reality.