Relapse

I find relapse to be an interesting subject in writing. Often, people see it as a failure to do good things. People relapse into drug abuse or delinquency, for example. But I see relapse more as two versions of one's self fighting for control over the body.

People really relapse into old habits. Old habits are usually bad ones because they are the only ones worth bothering to change. But an old habit could be a way of talking like, using certain words or phrases. An old habit could be a positive one like reading the daily news that you let slip by. Admittedly, "relapse" has such a negative connotation that it is rarely ever that it is applied to picking up good habits we once had.

Still, habits we have are part of a whole state of being. The person we used to be had the old habit and the person we are now has different habits. When old habits creep up again, the whole mindset tends to come with it, and in that, the struggle between the two selves plays in. This could be explored in several ways: the outside looking in (a friend watching it happen), the inside looking in (the person exploring this all happening), the inside looking out (the person not realizing it, but experiencing the world in the different ways). It could also be done in several genres, like in a very realistic way, or in a science fiction/horror setting where the two selves are more literally fighting for control of the body.

"Relapse" is a general term, but within that generality, it provides much fodder for creativity. Try exploring it and seeing if you can find an interesting story there. If not, research the subject a bit and try again.