Reversion.

Definition: a return to a previous state or practice.

Such a trait can be frustrating to deal with. Let's say you're prone to anger. You manage to not lose your temper for a day, only to blow up at everyone you see the very next day. Maybe next time, you stave it off for another day, or a week, or even a month, but it always comes back without fail. It is a constant companion for some.

If they're so irrevocably tied to it that it happens to them on a regular basis, is it even possible for them to improve? That's what Fluttershy is about to find out towards the end of her first quest for Breezies.