Late December is a peculiar psychological environment. There is distance from ordinary pressure, a natural boundary in the calendar, and a mild sense of possibility. In that state people make commitments that are structurally incompatible with the state they will actually be in on 14 February.
This is not a character failure. It is a design failure.
Three things go wrong reliably. The first is scale: the resolution is set at the size of the ambition rather than the size of a bad Tuesday. Anything that requires a good day to complete will not survive a bad month.
The second is that resolutions are almost always framed as outcomes — lose fifteen kilos, be more patient, spend more time with the family. Outcomes are not actions. You cannot do an outcome on Thursday morning. What you can do is a specific, small, unambiguous behaviour that you either did or did not do.
The third, and the one I see most in coaching, is that the resolution is aimed at a symptom while the generating pattern is left untouched. Somebody resolves to stop losing their temper with their children. The temper is not the problem; the fourteen-hour day that leaves nothing in reserve by seven in the evening is the problem. The resolution asks them to be a better person under identical conditions, which is not a plan.
If you want something to hold, do the opposite of what December encourages. Make it embarrassingly small. Make it a behaviour rather than a result. Attach it to something you already do without deciding. And spend your attention on the conditions that generate the pattern, rather than on the pattern itself.
One more thing, which matters more than any of the above: build in what happens when you miss. Everybody misses. The people who sustain change are not the ones who never break the chain — they are the ones who decided in advance that breaking it once is data rather than a verdict.