A speaker sits still and tries, almost as an exercise, to care about nothing. The word "endeavor" gives it away: letting go takes effort, so the calm is not quite effortless. The title, "Mindless," points the same way, as a state the speaker wants to reach, not one they already have.
The middle stanza is the strangest. The speaker compares themselves to a drunk in the "nasty end of a nasty room," and the word "vast" makes that image feel both sprawling and slightly absurd. It suggests a heavy, slumped, unguarded emptiness, a mind that has switched off without much dignity. There's a hint of self-mockery here, as if the speaker knows how grand and how grubby this idea of emptiness can be.
Then the light fades, and ordinary life taps on the shoulder: supper, the garden. The mood ends gentle and quietly humorous. Stillness is lovely, but the day's small, real pleasures are calling, and the speaker seems ready to answer.