Bonta's short poem seems to describe a long, wakeful night. The speaker imagines the night "falling" on them, and the bridge of the title becomes a way across to morning, something to be crossed rather than slept through. A troll is the creature that lives under a bridge, so the speaker may be picturing themselves in that role: guarding the crossing, stuck in the dark, waiting.
The second image is the most vulnerable one. The speaker's heart is "childish as a troll / in a paper palace," a grand home that is really flimsy and could tear at any moment. It suggests a feeling that is sincere but easily hurt, living inside something made up.
The last lines turn to lies. A lie can be used to build things, the poem says, but it can also be laughed down. The mood is wistful and a little wry. The speaker seems to know their palace is make-believe, and they can both rely on it and laugh at it.