Fall off the wagon.
[Idiom] FALL OFF THE WAGON. - Learn Everyday English Expressions with Ease
Resuming bad habits.
Fall off the horse" usually refers to someone who has resumed a bad habit they had previously given up, particularly in the context of addiction recovery like drinking alcohol or smoking. Imagine someone struggling to stay atop a metaphorical wagon of good habits; when they "fall off," they fail to maintain their sobriety or other positive behavior, just as someone might literally fall off a moving wagon and find it hard to get back on.
Example Sentences
He fell off the wagon.
He relapsed after a period of improvement.
She fell off the wagon after months of staying sober.
She relapsed after months of sobriety.
It can be difficult to get back on track once you fall off the wagon.
It is hard to get back on track after a relapse.