A new biography reveals more details about the Letterman sex scandal and his resulting depression

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David Letterman was swamped in scandal in 2009, when CBS News producer Robert “Joe” Halderman attempted to blackmail him after coming across a diary that proved Letterman had been having an affair with his assistant, Stephanie Birkitt.

Now a new biography, Letterman: The Last Giant of Late Night by New York Times reporter Jason Zinoman obtained by PEOPLE, reveals what was known by the Late Night host’s inner circle, and how the scandal sent him into a deep depression.

” ‘I’m in hell. I will always be in hell until the day after, when I will go to hell,’ ” Letterman reportedly said to writer Steve Young when they were going over the monologue in which he would announce the scandal on his show.

After revealing the scandal (“I have had sex with women who work for me,” he said on air), the host was “shaken.”

” ‘It was akin to having killed your family in a car crash. It was like that to me,’ ” Letterman said about his desperation, according to Zinoman. ” ‘I was afraid my family was gone.’ ”

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In an interview with Rolling Stone in 2015, Letterman revealed he was grateful that his wife stayed by his side and that he had yet to explain the situation to his son, Harry.

“He will [know about it] one day,” Letterman said. “We’ll have to have a conversation about it. But not yet.”

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Just as it was news to Letterman’s family, the book reveals that many of his staff members were also unaware.

“Most Late Showstaffers said they had no idea that he was having an affair with Birkitt,” writes Zinoman. “Some suspected as much, and a few said they knew what Letterman was doing. There had been rumors among some on staff about Letterman’s flirtations for years.”

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Adding to the drama, when Letterman’s ex-girlfriend and former head writer, Merrill Markoe heard of the affair, her “mind reeled.” She told Zinoman that Letterman had once asked her to write lines for Birkitt. To summarize: Letterman wanted his ex-girlfriend to write lines for his lover, while he was married to Regina Lasko (the woman with whom he’d cheated on Markoe with).

On her blog, Markoe joked: “As you can imagine this is a very emotional moment for me because Dave promised me many times that I was the only woman he would ever cheat on.”

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Facing criticism from all sides (in a 2015 NYT interview, Letterman reflected on the affair and said that CBS “had good reason to fire” him), the host was in turmoil.

According to the biography, after his confession, Letterman organized a postmortem meeting with his staffers, but no one “was allowed to talk about the show.” As Zinoman explains it, rather than a postmortem, the meeting was Letterman’s way “to cope with and avoid his personal life.”

” ‘I was looking for a refuge,’ ” Letterman said, according to Zinoman. ” ‘Whether they knew it or not, [my staffers] were being used to support me. I didn’t want to go outside. Outside I was scared. Scared as I’ve ever been in my life. The show was endlessly helpful.’ ”