The phrase "painful experience" refers to a time when something was physically or emotionally difficult or caused discomfort and unhappiness. Full definition
The feeling of loneliness is one of the most painful experiences of most people; the number of complaints about the loneliness is growing every year, and especially in the big cities. (russianwomen.club)
It can be a very painful experience for the child, to feel unwanted especially since they aren't able to wrap their head around the concept of failed marriages. (marriage.com)
It was one of the most painful experiences of my life. (inc.com)