Social anxiety runs through every stanza here, and "Aunty Annie" seems to stand for the inner voice that feeds it. Judging by the imagery, she is less a real relative than a comforting, controlling presence who offers warmth but keeps the speaker in the dark, telling her to turn off the lights and stay quiet.
The middle stanzas play out the speaker's racing thoughts. She second-guesses a simple greeting, wonders whether someone dislikes her or could ever love her, then tries to stop herself. When people walk toward her, she imagines them judging her as a "nerd" before she has said a word. Her body joins in with sweaty palms and gnashing teeth.
By the end she is holding back touch, speech, and any hint of attention. The mood is tense and self-conscious, then resigned. The final lines suggest she has accepted the darkness as where she and her thoughts "will always be," which feels both safe and sad.