Verb
fall off the wagon (third-person singular simple present falls off the wagon, present participle falling off the wagon, simple past fell off the wagon, past participle fallen off the wagon)
(idiomatic) To cease or fail at a regimen of self-improvement or reform; to lapse back into an old habit or addiction.
Though he fell off the wagon several times, he eventually succeeded in quitting.
Once you get into a routine of eating healthy, it hurts twice as much when you fall off the wagon. Carrie Underwood
I'm not a machine. I get really motivated, then I fall off the wagon and want to eat Chinese food and sit on my couch and gain five or 10 pounds! Shemar Moore