A routine bends, a rule breaks, and then it is gone

The difference between the two is what happens on the day you do not do it.

A rule has a binary quality. It is either kept or broken, and once it is broken it has lost the property that made it a rule. This is why people who set themselves strict daily commitments so often abandon them entirely after a single miss. The miss did not cost much in itself. What it cost was the perfect record, and the perfect record was the thing being maintained. With that gone, the whole arrangement is suddenly negotiable again, and the second miss follows the first much faster than the first followed the start.

A routine has no such property because it was never framed as a record. It is simply what usually happens. It can absorb a missed day the way a footpath absorbs a day with no walkers on it. Nothing is broken, nothing has to be restarted, and the next occasion is unaffected. The distinction is entirely in the framing, and the framing is chosen by you, usually without noticing that you are choosing it. Counting your streak converts a routine into a rule, which is why counting is a mixed blessing.

If you want the thing to last, describe it to yourself in a way that survives an exception. What you usually do. What normally happens after dinner. How mornings tend to go. This vocabulary sounds soft and it is structurally much tougher, because there is no state it can be knocked out of. Precision about the action is useful. Precision about the record is a liability.