Rosy retrospection
Rosy retrospection is when we remember the past as better than it really was. Old times are idealized; the difficulties and disappointments we actually felt are downplayed or forgotten. It can make us nostalgic for periods that were mixed or hard, and can distort our decisions when we compare the present to an idealized past.
Examples
You remember your school days as mostly happy and carefree, forgetting the stress and boredom you felt at the time.
An old job is recalled as rewarding and fun, while the daily frustrations and reasons you left are faded.
A past relationship is remembered for its best moments; the arguments and incompatibilities are minimized.
"The old days" in your town or country are remembered as safer and more civil, even when data or your own diary suggest otherwise.