The speaker is disgusted that a person who is now dead, a "rotting corpse" buried underground, still controls how they live. The poem opens and closes with the same line, "it's sickening," which makes the feeling seem stuck on repeat, just like the speaker's inability to move on.
The middle lines show what that hold looks like. The speaker tries to burn neatly, to scream quietly, to bleed without staining the carpet. These images of fire, drowning and bleeding are violent, yet the speaker is busy being careful and tidy about them. That suggests they learned to hide their pain and keep their suffering from inconveniencing anyone. The poem says these are "lessons" taken from the dead person, though it never says who that was or what happened.
The mood is bitter, cold and exhausted. The short, clipped lines and the contrast between the buried body and the speaker's ongoing struggle make the anger feel quiet but heavy.