The speaker is waiting for a relationship to end, and has already rehearsed how it will hurt. Nothing has happened yet. The whole poem sits in the "when" of what's coming: the stopwatch running out, the bags stacked by the door, the line about things "not working out." That makes it feel less like a goodbye and more like dread.
The imagery is cold and practical. A stopwatch suggests the relationship has been on a countdown, and the neatly stacked bags make the departure sound organized, almost businesslike. The bandaid image is interesting too. Ripping one off quickly is supposed to be the kinder way, yet here it still stings, and the lights going off along with it adds to the sense of finality.
The mood is quiet, resigned and sad. The speaker says they've "prepared," but the last line, with the heart dissolving in darkness as footsteps fade, suggests that no amount of preparing really helps. The sound of leaving is what lingers.