She was screwing a married man, only cared about her "status"/how she looked to others, and left her kid with a guy she barely knew so she could go to some stupid baby shower (that she probably could have taken her kid to anyway).
Her life sucked, yes, but she sucked too.
Would you like some hot coffee - in your face?!!! - Bert Macklin, FBI
I didn't particularly like the movie (but for other reasons) but your complaint flies in the face of some of the rules of storytelling. The whole idea is to end things with your characters better off in the third act. Amy Adams character was different by the end of the film having ended the affair, had rejected her perceived need for societal status, and accepted that the ultimate good in her life came from her family, not outside of it.
My problem with the film was the pacing and the way they told the story. They were trying to throw in extra plot devices for the sole reason that they could tie them into something profound (like the retarded CB radio scene) at the end of the film. It showed that the writers didn't have the courage or belief that their story would resonate with the audience without those devices.
Touch darkness and darkness touches you back.