In late 2013, Hoffman opened up to his children about his situation and went to rehab.
He stayed sober for three months, O'Donnell said, but his problems resurfaced when he went to Atlanta to film The Hunger Games.
"For the first time I realised that his addiction was bigger than either of us. I bowed my head and thought, I can't fix this. It was the moment that I let go," she said. "I told him, 'I can't monitor you all the time. I love you, I'm here for you, and I'll always be here for you. But I can't save you."'
Hoffman had moved around the corner from O'Donnell and their kids in New York City, close enough that he could still walk them to school.
She said she was hopeful when he returned from Atlanta as he had promised to go back to rehab.
"It happened so quickly. Phil came home from Atlanta, and I called a few people and said that we needed to keep an eye on him. Then he started using again, and three days later he was dead," she said. "The circumstances of Phil's death were so public — people around the world knew he was dead an hour after I did — and every detail, from the days leading up to his overdose to his funeral, were, and remain, all over the internet. And so I need to keep the rest of that awful time private."
"I had been expecting him to die since the day he started using again, but when it finally happened it hit me with brutal force. I wasn't prepared. There was no sense of peace or relief, just ferocious pain and overwhelming loss. The most difficult — the impossible — thing was thinking, How do I tell my kids that their dad just died? What are the words?"
O'Donnell said she finally agreed to get a family dog this year, despite not being a dog person.
"The moment we made the decision, Cooper said, 'She's going to die. Dogs don't live very long, so we're going to see her die.'"
"In her birth and in her coming to us, we were also mourning her death. Something about that felt right, knowing that everything you meet or love is going to die. I was in awe of my kids that they were able to hold both things in their heads at the same time. That's who they are now. And it hasn't stopped them from loving this little creature (her name is Puddles) scampering around our apartment. None of them wants to hold back. They've given their hearts to her, without hesitation or reservation. They're all in," she said.