"Not Mine" is about unrequited love, told by a speaker who has fallen for someone who will never return her feelings. She opens with a list of maybes: maybe it was his voice, his eyes, his walk. That string of guesses shows how confused she is, unable to say exactly why she fell. She was too shy to speak, and the poem hints that another girl has since eased his pain, leaving the speaker with nothing but her own effort, which now feels wasted.
The feeling that runs through it is quiet heartbreak. She smiles falsely so he won't notice, and keeps repeating to her heart that he isn't hers, as if she has to teach herself the fact again each day. The mood is sad but gentle, with no anger in it. In the last stanza she turns to the moon as he walks past, and comforts herself with the hope that the pain will end soon. That hope sounds more like a wish than a certainty.