The poem is a short portrait of a girl, built from five lines that all begin the same way. Each repeats "A girl so..." and adds another quality: young, naive, full of life, made of light, holding nothing but hope. The repetition feels a bit like someone slowly turning over a photograph, noticing one detail after another.
The surprise comes at the very end, in the signature line: "Who I used to be." Suddenly we realize the speaker is describing their own younger self. That changes how the earlier lines read. Words like "naive" and "nothing but hope" now carry a quiet ache, as if the speaker knows what the girl didn't. The mood is tender and wistful, with a hint of sadness. The speaker doesn't say what changed, and the poem leaves that unspoken, so the sense of loss is felt rather than explained.