The speaker looks back on a long stretch of waiting, watching each day's tomorrow arrive and fade with a kind of disbelief. Time feels hollow, and meaning is only a possibility, something that might exist "in fields of being" rather than something the speaker can grasp.
The middle of the poem turns to a bleak image: reason dying on cold Greek marble, with hemlock going to seed. Hemlock suggests Socrates, so the lines seem to mourn the loss of philosophy and rational certainty, leaving only the empty idea of cold stone.
The ending moves to the desert, where life drags on and the shadow in the doorway stretches even longer. Yet the final lines shift slightly. The speaker keeps putting "form" in quotation marks, as if unsure it can be trusted, but then sees it as a doorway opening. The mood is desolate and uncertain, with a faint, hesitant hope that some shape or way forward may still appear.