Robin Williams' widow says it wasn't depression that killed the beloved actor

SAN FRANCISCO -- For the first time since her husband's death in Marin County, Robin Williams' widow opened up about the beloved actor. In an exclusive interview with ABC's Amy Robach, Susan Williams said it was not depression that killed Robin.

Robin and Susan had a seven-year relationship; a happy and very private three-year marriage. But on Aug. 11 last year, life took a drastic turn. Susan left for work that morning, thinking her husband was still asleep.

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"Rebecca, his assistant, came over and I said, 'Call me when he's up or have him call me when he's up,'" said Susan. "And then she went me a text. It said, 'He's not up yet. What should I do?'"

When asked if she got to see him, Susan said, "Yes, and I got to tell him, 'I forgive you with all my heart, you're the bravest man I've ever known.'"

Robin's symptoms began in November 2013, she said in an interview that aired Tuesday on ABC's "Good Morning America." They included stomach pain, constipation, urinary trouble and sleeplessness.

"In November of 2013 he had a little gut pain and we never found out what it was," she said. "And then it went to next month it was another symptom. It was like this endless parade of symptoms and not all of them would raise their heads at once."

By the following May, he was suffering from stiffness, slumping, a shuffling gait and "losing his ability in his voice," she said.

"It's one minute, totally lucid," she recalled. "And then, five minutes later, he would say something that wasn't - it didn't match."

His autopsy later revealing he was suffering from a debilitating brain disorder.

"Lewy body dementia is a complex disorder with many different presentations. You can see very dramatic effects with thinking, emotions, and behavior," said Dr. Dan Kaufer, Director of UNC Memory Disorders Program.

Amy: "Most people think your husband killed himself because he was depressed."

Susan: "No, Lewy body dementia killed Robin. It's what took his life. And that's what I've spent the last year trying to get to the bottom of, what took my husband's life."

In what would be the final week of his life, doctors were planning to check him into a facility for neurocognitive testing. But in those last weeks, he was "disintegrating before my eyes," she said. "We were living a nightmare."

Robin, who had battled substance addiction in the past, was clean and sober when he died, she said, having recently marked eight years of sobriety.

Susan went on to say that Robin was losing his mind and he knew it.

"He was keeping it together as best he could," she said. "But the last month he could not. It was like the dam broke."

Susan described her husband as "just a dream" and their relationship "the best love I ever dreamed of."

More of the exclusive interview will air with Susan Williams will air on "Good Morning America" at 7 a.m. on ABC7, including her legal battle with Robin's children over his estate.

If you are or know somebody who is having thoughts of suicide, please call the National Suicide Prevention Lifeline at 1-800-273-8255. Click here for suicide and mental health resources.

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The Associated Press contributed to this report.