Probably Rachel/Cable since they grew up in apocalyptic futures.
But a lot of the X-Men had messed-up childhoods.
Jean felt her best friend due in her head at the age of 10, almost died herself and then spent a year in a coma
Storm's home got bombed, which killed her parents and left her with crippling claustrophobia and then she was trained as a thief in Cairo .
Scott had to leave his parents in a burning plane and jump with his younger brother. He then was put in a orphanage secretly run by Mister Sinister who had a young clone (I think?) called Nathan that bullied him all whilswhilst dealing with uncontrollable blasts that came from his eyes
I could go on...