The speaker admits to something that sounds quieter and more worrying than ordinary sadness: they've stopped feeling altogether. The first line is flat and blunt, and the next two set up a contrast. Once, there was sadness, which at least meant something was happening inside. Now that has gone, and what's left is only "numb."
The way the poem looks on the page does a lot of the work. The sentence breaks off at "just," and then the final word sits far off to the right, isolated in empty space. That gap works like a pause, or a long, tired breath before the speaker finds the word. It also makes the numbness feel like distance, as if the speaker is stranded away from everything else.
The mood is hollow and still. Nothing is dramatic here, and that's the point: the poem doesn't cry out, it simply states. The plain, short lines make the emptiness feel real, and a little unsettling, because the speaker no longer seems to have the energy to be upset about it.