The poem follows a girl named Eli as her sixteenth birthday approaches. She has always been a dreamer, and her dreams hurt her, because imagining who she could be and what relationships she could have only highlights how far real life falls short. She insists she doesn't care about her birthday, but the poem quickly catches her out: "but did she mean it? no." She cares enormously.
The mood is quiet, lonely, and quietly heartbreaking. The days before the birthday drag, and she expects to cry and fight. The "sweet sixteen" she pictured as a child turns into crying alone in an empty room and sleeping with ice on her eyes, presumably to hide the swelling. Nobody can know, because she feels she has to seem like the perfect girl. The question "wasn't she?" suggests she's unsure whether that image is true, or whether anyone sees past it. The final line, "happy sweet sixteen eli," reads as bitter, as if the speaker is wishing herself a happy birthday because no one else will.