The habit that survives a busy day usually has a version small enough to begin.

Habits fail on the worst days, and the worst days decide whether a habit lasts. Anything needing an hour, a mood and full equipment disappears the moment a week turns difficult, and after two missed days most people stop altogether. A version small enough to do while tired keeps the thread unbroken. Continuity beats intensity over any length of time.

Define the smallest honest version of your habit: two pages, five minutes, one set. Use it on the bad days instead of skipping and starting over.