In The Great Gatsby, what does Myrtle mean by "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.

What is ironic about Myrtle's statement, "you can't live forever", in The Great Gatsby?

Myrtle’s statement is ironic but not just because she is killed later in the book. If anything, the book shows that she is speaking the literal truth. What irony there is in her statement stems from the context in which she makes this claim. Myrtle is telling Nick about how she first met Tom:

“It was on the two little seats facing each other that are always the last ones left on the train. I was going up to New York to see my sister and spend the night. 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. When we came into the station he was next to me, and his white shirt-front pressed against my arm, and so I told him I’d have to call a policeman, but he knew I lied. I was so excited that when I got into a taxi with him I didn’t hardly know I wasn’t getting into a subway train. All I kept thinking about, over and over, was ‘You can’t live forever; you can’t live forever.’”

Her expression is meant to convey how going with Tom was a desperate, once in a lifetime chance. In fact, it’s easy to see that Myrtle was pretty calculating in her approach to Tom; she “couldn’t keep her eyes off him” precisely because she hoped to attract his notice. Far from taking a desperate chance, Myrtle knew exactly what she was doing. She doesn’t mind at all being “kept” – she has no problem spending Tom’s money, and it is only when she tries to exert power over Tom’s emotional life – their fight over whether she should talk about Daisy – that she feels Tom’s real power over her (he breaks her nose). Myrtle’s statement about “not living forever” also suggests a “live for the day” mentality that belies the reality of her situation. That is, rather than forsaking duty to her husband for a life of immediate pleasure, she has exchanged one kind of misery for another.

It’s also worth pointing out the similarity between Myrtle’s “not living forever” and Gatsby’s desire to relive the past. One the one hand, Myrtle is all too willing to forget her past and throw in with Tom, while on the other hand Gatsby would sacrifice the wealth Myrtle craves for a return to his past with Daisy. In this case, the irony stems from the fact that either approach is doomed to failure; both Gatsby and Myrtle are grasping after something that leads, ultimately, to their deaths.