For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Amory Blaine grows up in Minnesota, where his snobby mother raises him to think that he's better than every other person in the world. But Amory isn't in school long before he realizes he'll have to hide his sense of superiority if he wants to fit in and make buddies.
It takes a lot of effort, but he somehow manages to get through his early school years without becoming a total outcast. He eventually goes away to school and becomes his own person (sort of) after his mother dies. But now he has to deal with the problem of deciding what he's going to do with his life. Because let's face it; freedom can be freaking terrifying.
Act II
Amory doesn't do so well at school. But he's always been naturally intelligent enough to just coast through life, so he doesn't try any harder than usual and he ends up flunking a crucial class. He wants to be successful in life, but has no interest in working hard to achieve it; so he just keeps coasting and thinking about what to do with himself… until World War I answers the question for him. He's going off to fight, and that's that.
Act III
Luckily, Amory manages to make it out of World War I alive, although many of his friends die in the conflict. Back at home, he's more aimless than ever. He falls deeply in love with a girl named Rosalind and plans to marry her. But she dumps him because he's not a big enough success.
Amory is crushed by the news and becomes an alcoholic to cope with his pain. His alcoholism eventually gets a bit better, but he never really finds an answer to the question of what he should do with his life. And yeah, that's pretty much how the book ends: with Amory still wondering.