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It's a retelling of the traditional Cinderella story. Her tone is ironic; she's mocking the "rags to riches" and "happily ever after" stories that never come true and never disappear.
Poetic devices include repetition ("That story."), simile ("the white truck like an ambulance"), and symbolism. The white dove at first symbolizes her dead mother's love, and it's gentle and loving, but by the end of the poem it's become violent, pecking out the eyes of the stepsisters. The way the stepsisters slice up their feet for the prince symbolizes "the things we do for love" that we shouldn't do. (Sexton would probably have said it symbolizes what women do for men, but I think it goes both ways.) And the entire Cinderella story symbolizes our yearning for the solution that will end all our troubles and keep us permanently happy.
Hope this helps. Read the poem and ask yourself what it means to you, how you feel about the classic Cinderella story, how you feel about her version of it - that will help you sort out what she's saying. Good luck.