The peak-end rule describes how our mind evaluates an experience: not as the average of every moment, but mainly by its emotional peak (the most intense point, positive or negative) and by how it ended. Whatever happened in between often fades.
The classic example is a holiday or a celebration: you may have spent hours that were unremarkable, but if the day held one wonderful moment and closed beautifully, you'll remember it as excellent. Conversely, a trip that went well can "sour" in your memory if the final minutes were stressful, say an argument at the airport.
Psychologically, this explains why the story we tell about our own lives isn't always fair. We may judge entire periods or relationships by their difficult ending, forgetting the good that was there. A bitter goodbye can retroactively colour years of genuine value.
Knowing this rule helps you be kinder to your memories and more mindful of your "endings" — of a day, a conversation, a season of life. If you feel your past haunts you through a few intense moments, working through it with a professional can help; see our services.