The "Ghost" star relapsed during a trip away with Ashton, and the actress claims she was told by her then-spouse that alcoholism wasn't a "thing".
Moore said that it was one of the lowest points in her life.
"I think also when she wanted to have another baby and then it wasn't happening, and there was so much focus on that, it was like, 'Oh, we're not enough, '" Rumer explained.
Demi also shares daughter Scout Willis, 28, with ex Bruce Willis. "I made my own story up that [Ashton] wanted somebody that he could have wine with and do stuff with".
Appearing on the Red Table Talk, Moore's youngest daughter spoke about her relapse and how it impacted their lives.
The former couple eventually conceived a daughter, but Moore miscarried at six months, and when she struggled to fall pregnant again, she suffered a relapse in her longtime battle with substance abuse.
Her youngest daughter Tallulah was just nine-years-old at the time and took the opportunity to speak up on the impact her mother's relationship had on her growing up - especially after her older sisters moved out. "I never once went to you and said, 'I'm so sorry" [or] 'Are you okay?'" Rumer noted.
"I was there in the other room with 911 panicking because I'm like, either my mom is gonna die and I'm not gonna be in the room and I'm gonna feel the guilt of that for the rest of my life, or I'm gonna be there and see this image of my mom that I will never get out of my head".
She said: 'Scout came in to tell me and I had taken a bunch of codeine and I had done a bunch of cocaine that morning and Scout couldn't wake me up.
"Watching the behaviour with Ashton those years, because everyone had left the house and it was just me living there". Being like, please get away from me. "I remember there was just the anxiety that would come up in my body when I could sense her eyes shutting a little more or the way she was speaking or she would be a lot more affectionate with me if she wasn't sober", she adds. "It was not the mother that we had grown up with". "And I truly believed it", Tallulah said. "I'm like, 'I don't know who this is and they are supposed to be my rock'". But there was a point when I decided who she was, and in that moment, when I decided who she was, I realized that I had limited her from ever becoming anything else.
Recalling her own childhood, Moore made the decision not to do the same to her daughters, and with their support, dedicated herself to recovery.