Someone is speaking to a person in deep pain, begging them to get through the night. The speaker doesn't lecture or offer easy comfort. Instead they say "I know" again and again: I know what happens in your head, I know it's getting hard to fight. That repetition feels like a hand held out, a way of saying you're understood, not judged.
The word "swear" returns over and over, almost like a chant. It turns the poem into a promise the speaker is asking for, and the pleading gets more urgent with each line. The image of a "light" that might go out is simple but heavy, and the "voices" and the "darkness you fed" suggest a mind turned against itself. The final line shifts from asking to believing: "I know you'll swear to live instead." The mood is tense and frightened, but also tender, and it ends on fragile hope rather than certainty.