Use fresh starts

Researchers Hengchen Dai, Katherine Milkman and Jason Riis found what they called the fresh start effect: people are more likely to start new goals at the beginning of new periods, such as a new week, month, term, birthday or after a holiday, because these moments feel like a clean break from past failures.

You can use this deliberately. If you fall off track, pick the next Monday or the start of next month as a restart point, and plan it in advance: what you will do, when and with whom. You do not need to wait for New Year. But a fresh start works only if you plan the first small step, not only the feeling.