Just scale it down a bit. Maybe say that you will drink only one cup of coffee every day and not cut it out completely. Act on your plan. No matter what your goal is, do something about it. This time of year brings financial worry and family tension, among other things.
People often sense that a co-worker is having a problem before managers do.
The first step in helping is taking note of observable changes in the co-worker. Performance deficits . Increased mistakes, inconsistent work quality. Attendance problems . Tardiness or absences.