Playing music, the speaker suggests, is a kind of dying. A song is "fatal" because each note disappears the moment it is sounded, leaving nothing behind, "as gone as ash." That image makes music feel like something burned rather than built.
The second stanza turns to a need. The speaker has to "meet myself at the harp," as if the instrument is the one place where they can face who they really are. Yet the word "who" hangs over the lines, and the sentence seems to ask a quiet question: who, with this need, could simply take time for granted?
The last image is the sharpest. Time is "knocking nails," a sound like hammering, perhaps a clock's steady tick, perhaps a coffin being closed. The title, "Marking time," fits both meanings: keeping a beat and waiting as life passes.
The mood is hushed, a little somber, but not despairing. Music is loved precisely because it can't last.