Wiktionary
fall off the wagon
vb. (context idiomatic English) To cease or fail at a regimen of self-improvement or reform; to lapse back into an old habit or addiction.
Usage examples of "fall off the wagon".
We'll take you, but she says if you fall off the wagon, you're gone.
Why, then, did she feel so panicky, as if the smell of adventure was going to make her fall off the wagon?