Fall Off The Wagon Meaning

(idiomatic) To cease or fail at a regimen of self-improvement or reform; to lapse back into an old habit or addiction.

Example:   Though he fell off the wagon several times, he eventually succeeded in quitting.
2014 August 11, w:Dave Itzkoff, "Robin Williams, Oscar-Winning Comedian, Dies at 63 in Suspected Suicide," New York Times
  In 2006, he checked himself into the Hazelden center in Springbrook, Ore., to be treated for an addiction to alcohol, having fallen off the wagon after some 20 years of sobriety.