How do you build a habit that actually lasts beyond three weeks?

The 21 day figure is a myth — research suggests a wide range, commonly around two months and much longer for harder habits. What predicts survival is not duration but design: a specific trigger rather than a vague intention, a size small enough to do on your worst day, and a defined minimum version that still counts. Attach it to something already automatic. Track it visibly. And plan for the miss, since the failure point is almost never the first lapse but the second consecutive one. Expect it to feel effortful for longer than motivational content suggests.