Lily feels unfulfilled. Her life is boring and she expects that it will get even more boring as it progresses. She looks to something better, and that something is Lawrence Selden.
This isn't exactly your typical dream stage, since Lily never fully commits to a life with Selden. She holds back because she always remembers that Selden doesn't have enough money to keep her happy.
At first, Lily was faced with a decision between being happy but "poor" with someone like Selden, or being miserable and rich with someone like Percy Gryce. As her social life begins crumbling at the hand of Gus Trenor and Bertha Dorset, Lily is stripped of both possibilities.
Lily sinks further and further into poverty, and further down the social ladder, and finally to the working class. Then, she's fired from her job. Lily despairs and turns to chloral (a drug to help her sleep) for comfort.
Lily overdoses on her chloral just when Selden was ready to rescue her from her misery.