The speaker of "Retiring" seems to be an older man looking back on his life and finding little left to hold onto. The title suggests both withdrawing for the night and withdrawing from life more generally, and the poem plays on both. He admits he is "too afraid / of any vice," which sounds less like virtue than timidity: he no longer dares to be reckless or indulgent, even if he might want to.
The second stanza turns to regret. It is "too late" to give "her" something, and the phrase "that old gone heaven" suggests a happiness or love that has slipped away for good. We don't learn who she is, and the poem leaves that blank, which makes the loss feel private and unresolved.
The last image is the bleakest: a plain meal of mutton, eaten alone in the dark. The mood is quiet, resigned, and a little sad, with a dry, bitter humor in how small his pleasures have become.