The difference between the literal meaning of the words and the author's meaning can be summed up in one word: irony.
By that I mean, this girl was probably told these things. She was urged to exercise, to diet, etc.
However, given the way this poem came to a "happy ending" that was anything but, this line should be read as anything but literal. We should hear the author whispering behind those words, "Exercise (for others, not yourself, and die anyway)" and "diet (in an unhealthy fashion, one that's forced on you)" and so on.