Sonder is defined as the profound feeling of realizing that everyone, including strangers passing by on the street, has a life as complex and vivid as your own. They experience hopes, dreams, friendships, routines, worries and an inner life, all of which you'll likely never know about or fully understand.
The term was coined by John Koenig in The Dictionary of Obscure Sorrows, a compendium of newly invented words for powerful feelings that don't have a descriptive term in the English language.
I had a sponser do me a favor once and tell me "you're a special case of the same old thing." It helped me get over myself.