Fading affect bias
Over time, the feelings attached to unpleasant memories fade faster than the feelings attached to pleasant ones. This helps us cope and move on, but it can make past hardships seem milder than they really were.
In everyday life
A year after a painful breakup you can talk about it calmly, while a happy trip from the same year still makes you smile.
You sign up for a second marathon because the misery of the final miles last time now feels distant, while the thrill of crossing the finish line still feels fresh.
Why your mind does this
We could never store the details of every person and event we meet, so the brain boils experience down into categories and rough summaries. These let us make fast guesses about new people and situations with little effort. Holding on to bad experiences, while letting old pain soften, also helped us learn from danger without being crushed by it.
Its family, in 30 seconds: Generalities
How to spot it
Notice when you look back on a hard time and think 'it wasn't that bad' just as you are deciding whether to repeat it.
How to counter it
Before repeating a choice that hurt last time, reread notes you made while you were living through it.