The speaker is in an old, empty furnished room, going through drug withdrawal, and the woman who was with him is gone. The title tells us the setting and the ordeal, and the opening line, "Now let's line out agony," sounds like he is steadying himself to face the pain one step at a time. The room is "bare / of schmeck," a slang word that, judging by the context, points to the drug he no longer has.
Then the poem widens. Her leaving becomes part of a "cataract of abstract force" that sweeps over everyone and pushes us into "leaps we don't control." The speaker feels at the mercy of something much bigger than himself. The only thing he claims as his own is "our attitude."
Even that grip weakens in the last lines, as his "mental anchor slips / from the muck of time." The mood is raw, dizzy, and lonely. The short, broken lines mimic a mind coming loose, while the speaker still tries to find some thread of control.