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
Lily thinks she wants to marry Gryce, but Selden's arrival complicates her decision. When she decides to go on a walk with Selden, she sacrifices her chances with Percy. At Jack's wedding, this loss is confirmed with the announcement of Percy's engagement to Evie.
Act II
Interactions with Gus Trenor become increasingly strained as Lily tries to juggle his ego with social propriety. She finds herself having to make nice with men like Rosedale, and, all the while, Selden lurks in the background. Act II ends with the tumultuous conclusion to Book I, when Lily finds herself compromised by Trenor and experiences a moral awakening.
Act III
Matters go from bad to worse as we inch toward our conclusion: death. Lily's social demise in its various stages is simply leading up to her physical demise at the end of the novel.