Sara opens like a fairy tale, with a happy child who loves the world and is loved in return. The tone shifts once she "discovers the truth" and finds the world harsher than she believed. Rather than accept it, she retreats into a world of her own and shouts at everyone else to keep reality away. From there the poem charts a slow collapse: her parents watch her turn into "something terrible," their love fades, and they leave.
The speaker's feelings build toward the last two lines, when she reveals the girl was herself. What she seems to feel is grief, shame, and loneliness. She says she only wanted to be "real" but felt surrounded by hate. Calling her old self dead and blaming a "monster" suggests she sees her present self as something that replaced the child she was. The mood is bleak and confessional. The simple, storybook language makes the sadness feel even more raw, like a bedtime story that goes wrong.