A dream feels so vivid that the speaker can't let it go. Waking up brings a kind of grief: the line "My happiness continued to sleep" suggests that the joy stayed behind in the dream, and the speaker is left without it. She tries again and again to return to that "beautiful world," but never manages it. Slowly, doubt creeps in, and she starts to wonder whether the place was ever more than imagination.
Then comes the turning point. She finds a feather that belongs to the boy from the dream, and it works as physical proof. Her happiness, which had been "asleep," finally "awoke." The poem plays with that sleeping and waking idea, so the dream and the waking world swap places by the end.
The mood begins in longing and uncertainty, then shifts to relief and quiet wonder. The simple, short lines make the final certainty feel gentle but firm.