Why does everything feel harder in the weeks after a big achievement?

It is common enough to be almost predictable. Several things contribute: the sustained arousal and focus that carried you through drops away, the structure and clear purpose disappear, the anticipated feeling rarely matches expectation, and identity briefly loses its anchor. This shows up after exams, weddings, launches, thesis submissions and job changes. It usually resolves within a few weeks. What helps is expecting it rather than reading it as a sign something is wrong, keeping some structure in place through the transition, and planning a modest next thing rather than nothing at all.