Here is a speaker who feels betrayed, not by one person, but by the whole world. The poem opens with a curse, and it is a blunt one. The anger is aimed at "the people" who encouraged the speaker to believe in magic, love and butterfly kisses. Those are the things we tend to be told about when we're young: wonder, romance, small tender gestures. The speaker once accepted them as real, and now seems to feel that trust was a mistake.
What makes the poem sting is the contrast. The first lines are harsh, but the last one softens into something sweet and almost childlike, trailing off with an ellipsis. That pause suggests the speaker can't quite let go of those memories, even while cursing them. The anger sits right next to longing.
The mood is bitter but also sad. Judging by the title, "Believe," belief itself is at the center: the speaker blames others for planting it, and perhaps mourns what is lost now that it's gone.