The speaker says they have made themselves "fine," the way someone might tidy a garden plot, but the comparison turns dark almost at once. The thing being prettified is a grave. Getting ready, grooming, putting on a good appearance: all of it starts to look like preparation for burial.
The last lines carry the sharpest twist. The speaker calls the grave an "estate" they lease out "to wormwood" every night. Wormwood is a bitter plant, and the word also carries a sense of bitterness and sorrow. So the body becomes a piece of property rented to decay, and the rent comes due nightly, which suggests sleep as a small rehearsal of death.
The mood is dry, wry, and a little chilling. The speaker doesn't wail or panic. They talk about mortality in the calm, businesslike language of real estate, and that flat tone makes the idea of vanity in the face of death land harder.