The speaker of "Lethargic" is worn down by a heaviness they hide from other people. The opening lines set up the gap: a smile on the outside, ice within. Nothing dramatic happens in the poem, and that is the point. The speaker lies under "great weight" and says that even if a sea flooded the room, they would do nothing. That is what lethargy feels like here, not sleepiness but a total loss of the urge to act.
The middle of the poem turns oddly gentle. Secrets are passed to a wren, a small bird, which suggests the speaker is cut off from people but still feels some tenderness toward the world. Giving "the devil his dirt" sounds grim, yet the speaker calls it "a touching mission," with a flicker of dry humor.
The closing image of the body changing "as a sea / high or low" brings back the sea from earlier, now as a picture of moods rising and falling. The tone is quiet, numb, and a little wry, more tired acceptance than despair.