Someone asks the speaker a hard question: what would you do if the person you love were gone? The answer is not really a plan. It is a list of fears. The speaker admits that love might fade, or that death might take the person too soon, and then describes what that emptiness would feel like.
The mood is heavy and aching. Ordinary things lose their meaning: the sun stops feeling warm, words turn into "white noise," songs become "eulogies," and photos become ghosts. The short lines make the grief feel blunt and stunned. The speaker also sounds a little betrayed, asking why the person broke their promise of "forever."
The ending softens things. Even in the darkest sky, one star remains, and it is the lost loved one. Grief stays, but so does a small, steady light of memory.