Backslide (verb)

To revert to a previous bad behavior or condition after making progress or improvement.

Origin:

From "back" and "slide" meaning to slide or slip back to a previous state or behavior.

Examples:

1. He promised he would quit smoking, but he ended up backsliding.
2. She had been doing so well with her diet, but she backslid and gained all the weight back.
3. He had gotten over his addiction, but he backslid and started using again.
4. She had been doing so well with her sobriety, but she backslid and started drinking again.
5. He had been saving money but he backslid and spent it all.