"It was a real marriage," she told Vanity Fair in its latest cover story. "It wasn't for the cameras. And it was a huge priority for me to stay in it. And that did not work."
Shortly after announcing their split in June, reports began to surface that Affleck had an affair with their nanny – claims he has adamantly denied. Garner opened up about the scandal, saying it had nothing to do with their decision to split.
"We had been separated for months before I ever heard about the nanny," she said. "She had nothing to do with our decision to divorce. She was not a part of the equation. Bad judgment? Yes."
The actress admits that one of the hardest parts of the ordeal was having to explain it to their three children.
"It's not great for your kids for [a nanny] to disappear from their lives," she said. "I have had to have conversations about the meaning of 'scandal.' "
Patrick Demarchelier / Vanity Fair
While it's been impossible for Garner to shield her family completely from the media coverage, she admits she's done her best to tune it out.
"I turned on CNN one day," she says, "and there we were. I just won't do it anymore. I took a silent oath with myself last summer to really stay offline. I am totally clueless about all of it."
But she's adamant that ignoring the outside attention doesn't mean she's not affected by it. "Ben says, 'Oh, you just don't care,' and I say, 'No, it's the opposite.' It hurts me so much, and I care so much," she says.
"I cannot be driven by the optics of this. I cannot let anger or hurt be my engine. I need to move with the big picture always on my mind, and the kids first and foremost."
Despite the turmoil, Garner says she still has feelings for the father of her children and has no regrets about marrying Affleck. "I didn't marry the big fat movie star; I married him," she explains.
"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. He's the love of my life.
"What am I going to do about that? He's the most brilliant person in any room, the most charismatic, the most generous. He's just a complicated guy. I always say, 'When his sun shines on you, you feel it.' But when the sun is shining elsewhere, it's cold. He can cast quite a shadow."
While those feelings continue to linger, Garner does seem to be gaining a sense of humor about the situation. Asked about her ex-husband's large phoenix back tattoo, she joked, "You know what we would say in my hometown about that? 'Bless his heart.' A phoenix rising from the ashes. Am I the ashes in this scenario?" she asks.
"I take umbrage. I refuse to be the ashes."