Boy, our speaker is none too fond of the tale she's telling (for convenience, we'll call her a she, since Anne Sexton wrote the poem). Just think of her as a kind of grumpy realist. She knows she's telling you a story you've heard before, but she'd like to inform you that it's nothing like what you think it is. Think of someone older (but maybe not much older) than you, who's seen the world and knows about all of its hardships—a well-traveled older cousin, maybe. The speaker may seem bitter and cynical, but we don't get the sense that she's deliberately trying to rain on your parade.
Instead, she tries to temper her gritty interpretation of what everyone assumes to be a happily-ever-after tale. How does she do it? Well, there are moments in the poem that are pretty funny, if in a sarcastic way. (Prince-as-shoe-salesman?! Come on.) So, it's not like the speaker is looking down on us for believing in that fairy tale mumbo jumbo. She's just coolly pointing out that, if you think about it, there is a darker side to the Disney movie version.