The speaker describes a harm done to themselves, and the title, "Self-inflicted," tells us so before the first line begins. The poem never says what the harm is. Instead it gives us an image: noon, the brightest point of the day, "turning into a bruise." Light darkens into something tender and discolored, as if the day itself has been struck. The line "I have given myself" is left hanging, which makes the admission feel quiet and a little stunned. The speaker owns the hurt without explaining it.
The second stanza turns to sound, or the lack of it. Something is "thin as thunder," a contradiction that makes the noise feel weak and hollow, and then comes "the fear of so much quiet." The speaker seems less afraid of loud pain than of the silence around it. Judging by the imagery, the mood is uneasy, bare, and suspended, like the stillness after something has happened and before anyone speaks.