"I drank relatively normally for a long time. What happened was that I started drinking more and more when my marriage was falling apart," he says. "This was 2015, 2016. My drinking, of course, created more marital problems."
Affleck calls his divorce from 47-year-old Garner -- with whom he shares three kids -- "the biggest regret" of his life, but notes he can no longer keep being ashamed of it.
"The biggest regret of my life is this divorce," he says bluntly. "Shame is really toxic. There is no positive byproduct of shame. It's just stewing in a toxic, hideous feeling of low self-worth and self-loathing."
"It's not particularly healthy for me to obsess over the failures -- the relapses -- and beat myself up," he continues. "I have certainly made mistakes. I have certainly done things that I regret. But you've got to pick yourself up, learn from it, learn some more, try to move forward."
"Relapse is embarrassing, obviously," he now says, referring to the incident. "I wish it didn't happen. I really wish it wasn't on the internet for my kids to see. Jen and I did our best to address it and be honest."
Later, Affleck discusses a pivotal scene in his upcoming film, The Way Back, which has parallels to his relationship with Garner. According to The New York Times, in the film, his character, Jack Cunningham, struggles with alcoholism and goes to rehab. He eventually apologizes to his ex-wife when she comes to visit him. "I failed you," Jack says. "I failed our marriage."
"It was really important, without being mawkish or false, that he make amends to her -- that he take accountability for the pain that he and only he has caused," Affleck reflects.
The film's director, Gavin O'Connor, says Affleck had a "total breakdown" after the emotional scene.
"It was like a floodgate opened up," O'Connor recalls. "It was startling and powerful. I think that was a very personal moment in the movie. I think that was him."
"I didn't marry a movie star; I married him," she said. "And I would go back and remake that decision. I ran down the beach to him, and I would again."
"You can't have these three babies and so much of what we had," she continued. "He's the love of my life. What am I going to say about that? He's the most brilliant person in any room, the most charismatic, the most generous."
Still, she acknowledged that Affleck was a "complicated guy."
"When his sun shines on you, you feel it. But when the sun is shining elsewhere, it's cold," she shared. "He can cast quite a shadow."
"The main thing is these kids -- and we're completely in line with what we hope for them," she added about their co-parenting. "Sure, I lost a dream of dancing with my husband at my daughter's wedding. But you should see their faces when he walks through the door. And if you see your kids love someone so purely and wholly, then you're going to be friends with that person."
"Ben's relationship with Jen is doing amazingly well despite everything that's happened," the source said. "She's really supportive of Ben, and when they parent the kids separately, they both share their stories of sweet moments they had with the kids that day. Their friends are always saying what a perfect example they are of dedicated parents who look past their own issues."