The speaker has been betrayed by someone they trusted, and the poem works through what that does to a person's sense of themselves. It opens with a string of near-proverbs that twist the same few words around: the trusting person is a fool, the fool is trusting. Then comes the defensive line, "I'm nobody's fool," as if the speaker is trying to convince themselves.
The middle section slows down. Trust is spelled out as a list of hopes: you're good, you won't hurt me, you find me interesting, you like me, you love me. The jump from "Likes me" to "Loves me" suggests the speaker was deeply invested. Then the blow lands quietly, in "what you did," which the poem never names. The speaker admits to clinging to other explanations for "a foolishly long time."
The ending is bitter. "Once I was somebody's fool at least" hints at a loss: being a fool meant being connected to someone. Now "nobody's fool" sounds less like pride and more like loneliness. The mood is wry, guarded, and sad.