Monkey On One's Back Meaning

(idiomatic) A state of persistent distress or worry or the cause of such a state.

Example: 1972 July 24, "The Blue-Collar Blues," Time:
  Prine's balladeering also includes social comment, as in Sam Stone, a song about a veteran returning from "the conflict overseas with a Purple Heart and a monkey on his back."
1949, Nelson Algren, "The Man With The Golden Arm":
  "You got my monkey, Dealer? You take my nice old monkey away from me? Is that monkey ridin' your back these days, Dealer?"
1922, D. H. Lawrence, "Wintry Peacock," in England, My England:
  "'E's got th' monkey on 'is back ower this letter job," said the father secretly to me.