Size it for your worst week
Pick a version so small it's absurd on a good day: two minutes of breathwork, one set, one page. On good days you'll do more anyway. On the bad week, the small version keeps the identity alive, which is the thing that actually compounds.
Anchor it to something that already happens
'After I pour coffee, I do five minutes of mobility' works. 'I'll do mobility in the morning' does not. Existing routines are the only reliable trigger you already own.
Never miss twice
One missed day is noise. Two in a row is the start of a new pattern. Make the rule mechanical and the guilt disappears — you're not failing, you're following the rule.
Make the evidence visible
A visible record — a tile grid, a checklist, a streak counter — turns an abstract intention into a thing you can see slipping. That visibility is most of the behavioural effect.