After quitting smoking for a year, he backslid into his old habits.
The student backslid into bad behavior after his parents stopped monitoring his actions.
After losing weight and getting fit, he backslid into an unhealthy lifestyle of fast food and no exercise.
She was making progress in therapy until she started backsliding into negative patterns of thinking.
She had been doing well in her recovery until she started backsliding into poor choices that led to a relapse.