Pro tip from the olds at Shmoop: if the best years of your life took place in high school, you're in for a long downhill slide. There's nothing wrong with remembering the good times, but living in the past just leads you to tragedy. (Or at least to being a major bore at parties.) In The Great Gatsby, living in the past is a lot direr than being boring. Characters pursue visions of the future that are determined by their pasts, which—in the memorable phrase that ends the book—makes us all into little boats beating against the current. And, unfortunately, some of those boats are doomed to sink.
Gatsby ends up dead because he can't live in the present—so he can't live at all.
Daisy, unlike Gatsby, is ultimately able to face reality and live in the present.