Can someone explain The Catcher In The Rye?

I used to love Cathcer in the Rye, due the raw and evocative writing style.
To answer what happens in the last chapter Holden was talking about all that happened in the previous chapters in past tense. The final chapter is in the present tense and he is speaking from an institution which helps his mental illness. He describes whatever he has as 'getting sick'. He is being psychoanalysed and next fall he will come out of the care centre and back into a school where he might actually apply himself this time, although this is ambigious. It is not clear whether Holden will try and get better and recover, or not. The final line 'don't every tell anybody anything. If you do, you start missing everbody' is showing that he is still scared and alone, just like he was throughout the entirety of the novel, although he is not so cynical anymore. He's allowed himself to gain some form of feelings for people and actually misses those who he has lost touch with due to his illness.
The novel, as a whole, is a lot about being alienated whilst growing up and how fake, or 'phony' the world really is. He struggles a great deal with lonliness, he isolates himself and has meaningless, almost one night, friends in order to satisfy some form of company. He keeps away from many people, including Jane, because he feels they could harm him so he'd rather shut himself away.
The loss of innocence is addressed in the above answer.
A lot of the novel is about Holden's individuality and how he strives to keep it. This is seen with the red hunting hat, a memorable symbol. He also undos a lot of relationships he almost has in order to preserve this side of him. This also leads to him lying to people, although this ironically makes him 'phony' which is something he hates.Basically the book is about
1) Alienation and loneliness
2) Growing up and loss of innocence that comes with it
3) Relationships and how they effect Holden's cynical attitude

Hope this helps