"She started partying, and she didn't really know how to stop," Cissy, 79, wrote of the Grammy-winner. "I want people to know the truth about her, how she really was."
Cissy said she wonders if she could have "saved her somehow," but admits the addiction grew so out-of-control, she didn't mention it out of fears Whitney would "run completely away" from her.
"What was the point?" she wrote.
As for Robyn Crawford -- the assistant some said Whitney had a lesbian relationship with -- Cissy said she knows "nothing about” a romance.
"I just didn’t want her with my daughter," Cissy said.
The I Wanna Dance With Somebody singer was slated to attend at a pre-Grammy Awards bash held by producer Clive Davis on February 11, 2012, the day she was found dead in her room at the Beverly Hilton. Officials later pronounced the cause of death as drowning, adding the singer had traces of cocaine in her system.
"I'm angry she died alone, in those conditions," Cissy said. "I'm still mad about that."
Remembering Whitney, published by Harper Collins, hits bookstores Tuesday.