Hans's "once" is a quiet confession of unreturned, or at least uncertain, love. The speaker tells the boy she likes him and immediately feels guilty, as if caring for someone who has "been someone's" is a kind of trespass. She even apologizes, saying her heart hurts because she likes him too much.
The center of the poem is a small, intimate act: waking in the night to pray for him. She calls it the most beautiful thing she has ever done for someone she loves. But then the mood shifts. She wishes he had done the same for her, and the last lines land with a sting: maybe he did, but for the girl he loves. The tone starts tender and shy, then turns wistful and bruised. What makes it affecting is how plain it is. She doesn't blame him. She simply admits how much she gives, and how little she can be sure of getting back.