EXCLUSIVE: Sean Astin Remembers Late Mother Patty Duke: 'She Was a Warrior'

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Sean Astin spoke with ET on Tuesday about his late mother, actress Anna “Patty Duke” Pearce, who died Tuesday morning at age 69.

Astin said that Duke was “surrounded by her family” prior to her death, after relatives traveled to her hospital bedside in Coeur D'Alene, Idaho. The 45-year-old actor described being with his mother in her final moments as “a powerful experience.”

“She was at peace,” he recalled. “When it was over, we all looked at each other and we thought, you can be an atheist, a skeptic, agnostic, you can be whatever you want, but what just happened in this room is indescribably beautiful.”

WATCH: Anna 'Patty Duke' Pearce, Oscar-Winning Actress, Dies at 69

Duke became a major advocate for mental health awareness after being diagnosed with bipolar disorder in 1982, a moment Astin described as a “watershed” for his mother’s life and legacy.

“All of a sudden, [here] was this thing that, not only was it important for her own life, because now she could design a caregiving plan, and she could get medication, and there was a name for what her series of symptoms was,” he explained. “But now, she had a new identity and a whole new mission. A whole new sense of purpose, and that was to share what she had gone through with other people.”

The actress, along with other advocates like Rosalynn Carter and Mariel Hemingway, spoke out publicly about her bipolar disorder and other psychiatric health issues at a time when mental illnesses carried an even heavier stigma.

“She was one of the first public figures with a big public profile who did that,” Astin recalled. “What's so funny is, as her son, seeing her -- the real her, the authentic her -- which most people saw as well... the more she opened up, the more she shared of her pain, the more she revealed how excruciating pain can be turned into bliss.”

“She just chartered a path,” he continued. “She would do fundraisers and she would lobby Congress and write another book specifically on bipolar disorder...She was indefatigable. Wherever I go on this planet, for my whole life, people would come up to me and they'd say to me, ‘How’s your mom?’”

WATCH: Melissa Gilbert Remembers Patty Duke After Her Death: 'She Was My Second Mom'

Astin reposted his family’s statement on his mother’s death on Tuesday, adding the simple caption, “I love you, Mom.” He also made sure to let friends, followers and fans know that Duke’s legacy of compassion and awareness will endure long after her death, posting a link to The Patty Duke Mental Health Project.

I love you mom.OUR FAMILY STATEMENT ON THE PASSING OF PATTY DUKEThis morning, our beloved wife, mother, grandmother,...

Posted by Sean Astin on Tuesday, March 29, 2016

“The story of her life is, there was always something challenging. Some physical malady, some mental thing, some financial thing, some career thing, some something,” Astin said of his mother. “The word I use to describe my mom when I write about her, when I talk about her, she was a warrior. My mom was a warrior in the grand tradition of the bravest, strongest warriors from ancient Greco-Roman history. My mother was a warrior.”

WATCH: Patty Duke Dies at 69 -- Hollywood Pays Tribute