"I was literally stunned," she said. "I just showed him the letter, and said, 'Can you tell me about this? Are you seeing her? What is this?' And he said, 'It's nothing, it's nothing, I'll end it.' ... And then we sat down, and he told me he was having an affair with this woman. It was incredibly painful for me."
Sanford said she gave her husband a chance to explain himself and decided to keep the affair a secret, hinging on his promises to end his relationship and change.
"My first gut instinct was to forgive him, not, not to kick him out. But, I told him immediately that I wanted to reconcile, that I would forgive him, but that it had to be over, and the marriage had to be much better. I wasn't going to go back to more of the same," she said.
Sanford said she was in the dark about the depth of his devotion. Even after Sanford learned of her husband's unfaithfulness, she said the governor pleaded with her to stay with him, but also begged for permission to visit Chapur, whom he called his "soul mate."
"I could never have imagined this. I mean, I could never have even made this up. It never occurred to me that this person I knew, who was actually a fairly grounded person, would be asking me something so morally offensive," Sanford said. "He said, 'Why, why can't you just give me permission?' I said, 'Well, why would I give you permission? I mean, who, who gives their spouse permission to go see their lover?'"
Sanford said her husband thought giving him the OK to see Chapur was a testament to her love, which she called a string of twisted logic.
"He said to me at one point, 'Why don't you just love me.' And I said, 'Well, that's not love. I mean, love has a responsibility,'" she said.
Sanford eventually gave in and Mark Sanford flew to New York with a friend to see Chapur, promising it would be the last time he saw her.
"It was awful," she said. "It just, it ripped me up. To think that he wanted to go be with his lover is just gut-wrenching. There were days when I was a puddle, and I think I was shaking at home, in the bed, just thinking about him even being with her. And the whole two -- I think it was a two-day trip -- I mean, I was nervous the whole time."
Mark Sanford never referred to her as his "soul mate," she told Walters.
By spring 2009, Sanford began to think the damage to their marriage was beyond repair, but continued to keep their marital problems a secret, for the sake of the children and for his political career. But the world was about to find out the truth through a bizarre chain of events.
"I knew he had been having an affair. I didn't know he was in Argentina," she said.
In the face of every political spouse's nightmare, Sanford opted not to stand by her husband's side during his June press conference when he confessed to cheating. Instead, she watched it on TV, like the rest of America.