off the wagon
(idiomatic) No longer maintaining a program of self-improvement or abstinence from an undesirable habit, especially drinking alcohol.
She kept up her diet for an entire month before falling off the wagon.
He is off the wagon again.
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