A habit becomes durable once it survives its first disruption.

Until something goes wrong, a habit is just a run of easy days. The first real disruption is the test, because that is when you find out whether the behaviour is attached to the conditions or to you. Getting through one interruption and continuing teaches your mind that a gap is survivable, and that lesson is what makes the habit stick.

So the day after you break a new habit is the important day, more important than any of the ones before it. Treat the return as the main event. Once you have recovered from one gap, the next one loses most of its power.