From Longman Dictionary of Contemporary Englishbackslideback‧slide /ˌbækˈslaɪd $ ˈbækslaɪd/ verb (past tense and past participle backslid /-slɪd/) [intransitive] BAD BEHAVIOUR OR ACTIONSto start doing the bad things that you used to do, after having improved your behaviour —backslider noun [countable]→ See Verb table
Examples from the Corpus
backslide• Many of the patients backslide into drug and alcohol abuse.• It would have looked, moreover, as though the Government were backsliding, so a swift change of tack was ordained. 