Three short lines, and they paint a small, rueful scene: someone has stayed in bed far too long, keeping company with their own "fool self." The phrase is half scolding, half affectionate, as if the speaker is both embarrassed by that self and resigned to sharing a bed with it.
The last line does the quiet work. "A bottle falling out" is left hanging, with no verb to settle it and no full stop. We can picture an empty bottle slipping from a hand or tumbling off the covers, which hints at drinking the night before, though the poem never says so outright. The title, "Slack," fits both ways: a loosened grip, a sagging day, a person who has let go of the usual effort.
The mood is sluggish, a bit sheepish, and oddly gentle. There's no big confession or drama. The speaker simply notices the mess and the lethargy, and the poem's brevity mirrors that unhurried, drifting state.