The night I had to put down my cat (he was the best boy).
Dealing with the aftermath of a family member’s suicide.
The day my twin first moved far away from me.
The death of my grandmother.
The time I was hospitalized against my will for a mental health issue.
The first panic attack I experienced and the subsequent fallout.
All said, I have lived a pretty lucky life and am a successful person (by my account). But even so, life is full of this type of stuff for pretty much everyone. Fun question, OP!