In The Great Gatsby by F. Scott Fitzgerald, what does Myrtle really mean when she tells Nick, "You can't live forever, you can't live forever"?
Myrtle is saying that line as a way of convincing herself that her actions are justified. The repetition of it is why I think that. It's Myrtle's version of "I think I can. I think I can."
At that point in the story, Myrtle is telling Nick how she came to meet and begin seeing Tom. She was on her way to visit her sister, when she saw Tom. She was very attracted to him, and Tom noticed.
He had on a dress suit and patent leather shoes and I couldn’t keep my eyes off him but every time he looked at me I had to pretend to be looking at the advertisement over his head.
Myrtle doesn't narrate what the two of them said to each other, but she says that the two of them left together in a taxi. That's when she told herself "You can't live forever."
The attitude is illustrating the concept of carpe diem. Mrytle is very unhappy with her current lot in life. She is poor, lives in the Valley of Ashes, and is married to a man that she no longer loves. Tom comes into her life and offers her a chance at the lifestyles of the rich and the famous. Myrtle knows that she might only get one chance with a man like Tom and the life that he offers, so she jumps at the opportunity, because she knows she can't live forever and get another chance.
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.