The actor and his wife of 13 years called time on their on/off marriage last year (10) and their divorce was finalised in July (10).
The former couple is parents to a daughter, Dylan, 19, and a son, Hopper, 17, who still lives at home, and Penn admits he would have liked to have given his son a stable upbringing until he graduates high school.
He tells the Hollywood Reporter, "A... profound change in life for me personally was not being able to raise my son in a whole family through high school."
Penn also reveals he lost half of his fortune in the divorce and although he would like to concentrate on his charity mission in Haiti, he has to get back to work.
He adds, "I got practical issues, like everybody... I had just got taken for one half of everything I had in the divorce, so it's not like I don't have to work... This (Haiti) is where I'll be when I'm not working, for the rest of my life."