A person needs time to stop preparing for the future they expected.

Much of daily life is quiet preparation for a future you assume is coming: money set aside, plans half made, a hundred small arrangements pointed at a particular version of next year. When that future ends, the arrangements do not stop instantly. You keep noticing things you were saving for it, and each noticing is a small separate loss. That is part of why this kind of grief takes longer than anyone expects.

Be patient with the reminders when they arrive, without treating them as a sign you are not coping. Change what has to be changed, and leave the rest until it feels possible.