Why a routine survives a bad week and a resolution does not

Resolutions depend on the state you are in. Routines depend on the situation you are in.

A resolution is a promise made by a version of you who felt capable. It is written in the language of intention and it is redeemed in the language of feeling: you will do this because you want to, and on the day, you need to still want to. A bad week attacks exactly that. It does not remove your ability to act, it removes the enthusiasm the promise was resting on, and the promise collapses because its only support has gone. Then the collapse is read as evidence about the person rather than evidence about the structure, which makes the next attempt harder than the first.

A routine rests on something more durable, which is context. It is attached to a time, a place, an object or another action, and those things are still there during a bad week. The kettle still boils. The bus still comes. The shoes are still by the door. When the prompt is external, the absence of enthusiasm matters much less, because you are not being asked to generate the impulse from nothing. You are being handed it by the situation, and you follow it in a slightly diminished way, and that is enough.

This suggests a useful test for anything you are trying to establish. Ask what happens to it on the worst plausible day. If the honest answer is that it depends on how you feel, the thing is a resolution wearing a routine's clothes, and it will hold until the first genuinely difficult stretch. If the answer is that it will happen, badly and briefly, because it is welded to something that occurs anyway, then you have built something with a chance of lasting. Durability lives in the attachment, not in the ambition.