Sarah's poem describes someone who hurts herself every night, in the hope of letting something terrible out. The speaker compares her to a butterfly breaking free of a cocoon, which makes the act sound almost like a transformation. A butterfly's escape is supposed to be beautiful. But the images that follow, tears mixed with pools of blood, show how much pain is really involved, and the comparison feels more like something the girl tells herself than the truth.
The "demons" seem to stand for her inner suffering, whatever she feels trapped with. She believes that opening her skin will release them, yet her skin is called "hollow," as if there is nothing left inside her but the pain. The repeated "every night" gives the poem a heavy, cyclical feel. The final line, "but the demons never left," lands like a door closing. The mood is bleak and exhausted, and it leaves the reader with a sense of how hopeless that cycle can be.