Every stanza of "Secret" begins with something the speaker has, whether a secret, a promise, a dream, a journey, a limit, or love, and then immediately asks a question she can't answer. The repetition of each opening line at the end makes the questions feel like they're going in circles, as if she's stuck thinking them over.
The middle lines keep shutting doors. She can't tell her loved ones because they'd gossip, she can't keep her promise because her tongue won't speak, and her dream exists only in sleep. The mood is lonely and uncertain, with a quiet ache underneath. The final stanza is the most revealing: she is in love, but "not with him, / But just his mind." Her love seems to be for who he is inside, not for the whole person, and she doesn't understand why. He is "elsewhere," so the longing and the confusion seem to belong together. The poem never settles anything, and that unresolved feeling is the point.