The speaker is describing a low, isolated state of mind, and they do it mostly through colour. Days are "grim," nights are "blue," memory is "tainted green," and vision has "lost its hue," as if the world has slowly drained of life. The one bright spot is an orange, whose juice offers a moment of sweetness. But even that slips away: the taste is something the speaker "can't manage," and the pleasure turns out to be a "mirage."
The poem suggests that too much of anything turns bitter, and the speaker's mood sours the same way. Glitter, a cheerful surface, seems to strip them bare rather than comfort them, which hints that shiny appearances feel false or exposing. The last line is the quietest and most telling: "I am the only witness." Whatever happens, no one else sees it. The mood is heavy, wary and lonely, with a small flicker of longing for something real to hold onto.