The speaker is stuck in a close, unhealthy bond with the "toxic mistress" in the mirror. She seems to stand for a darker side of the speaker's own mind, perhaps despair, addiction, or self-destructive thinking. The messy desk and empty stage suggest someone whose creative life or sense of purpose has stalled, while the darkness grows "with each passing note."
What makes the poem unsettling is how tender it sounds. The speaker cuddles this mistress, takes her promises "like sugar," and half-believes her "sweet lies," yet the question mark after the claim that wounds will heal shows doubt. She comforts and harms at once, creating the very mayhem she soothes. By the end she has erased the "warm fires," leaving only a storm in the speaker's mind and the grim sense that she will stay for good. The mood is weary, intimate, and quietly hopeless, with a hint of self-awareness that makes it sadder.