This is such a heartbreaking thread. Full of pain and such vulnerability too.
I have a few:
Watching my then 12/13 year old DS's face as he learned his Dad (my ex) had cancer, and again later when he started to realise his Dad wasn't going to recover. The way his childhood drained out of him in those moments and he was never the same again; young and carefree. And his subsequent MH issues. There is a saying I read on here once, about how a parent is only ever as happy as their unhappiest child, and I thought how true that was. The sheer horror of knowing your child is really suffering and struggling and not being able to do anything to take it away is the most painful and distressing thing in the world.
The absence of my Dad from my life growing up, and my mum (my only real family) being depressed and narcissistic, and alternating between being loving and fun and very scary/confusing. She used to tell me all the time she thought she would die young, but would 'try' and not die til I was an adult if possible, and so I spent most of my childhood in a state of terror and dread. The deep loneliness of seeing other families with average loving parents, and the knowledge I'll never have that is a source of deep grief to me. Some very traumatic events happened around me/to me as a child and left me feeling constantly, profoundly mistrusting of others and quietly unsafe all the time. I wish so much that wasn't the case.
My very good friend of 7 years ghosting me a few years ago and never really explaining why. I still miss him and dream of him often.
Never having a daughter, or having the experience of having a child with someone I stayed with in a couple and could bring them up side by side with, like a team. That family unit I longed so much for as a child.