Why does habit cue fail?

The most common reason habit cue fails: Trying to change the behavior without identifying the trigger first. You white-knuckle through willpower for a week, then the trigger fires when you're tired and the pattern returns at full strength. The pattern isn't the enemy. The unidentified trigger is.

The fix: Pick one pattern you want to change. Over the next three days, every time the behavior fires, immediately write down: (1) what time it is, (2) where you are, (3) who is around you, (4) what you were doing right before, (5) what emotion you were feeling. After three days, look at your logs. The trigger will be whatever shows up in at least three out of five entries.

The underlying principle is straightforward: Every pattern has a trigger — identifying the trigger is the key to changing the pattern.