In the west, we'd call that "dwelling in the past". It's not usually a good idea — but I would like to mention that "learning from the past" is an entirely different thing.

I've noticed that most people who advocate "not dwelling" also seem to disparage "learning from".

Good point. All Sufi aphhorisms have multiple levels of meaning, of course. some random thoughts from my end:

I saw this one as having special applicability to hatred. It's one thing to despise wickedness in passing - but the real danger to one's soul comes when becomes desirous of that feeling, and "the heart and mind chew on it endlessly." THIS is where a legitimate concept can become profoundly corrupting.

Where the mind and heart lead, actions will follow.
Sufis put a lot of focus on getting those first 2 right.

To link this to the Japanese monk story I referred to in my post, we have to learn to truly "put the lady down" when dealing with righteous hatred, doing what is necessary and right but then quickly ceasing to carry such things.

Of course, there's a wider application of Bushanja's ideas beyond hatred. A sin committed in passing may be serious indeed - but dark thoughts that become fixtures and fixations are unhealthy in the extreme, and can become the breeding grounds of true monsters.