What does Myrtle really mean when she tells Nick, "You can't live forever, you can't live forever"?

She says this to herself while debating having an affair
with Tom.  She realizes that life is beginning to pass her by and she is not getting any
younger.  She lives in the valley of ashes with her husband, George, whom she doesn't
love. And whom she believes led her to believe he was financially set.  When a man
knocks on her door one day asking for the suit he had borrowed to get married in, she
realizes he hadn't been forthright about his circumstances. She believes that by being
Tom's mistress, she will live the life she has always wanted for herself. Therefore,
it's a now or never decision which ironically ends up leading to her premature
death.