"It's not natural to outlive your child. This has always been my greatest fear," Reynolds wrote. "I don't know if I could survive that."
"Carrie is my child and I love her with every ounce of strength I possess," she added.
Reynolds died on Wednesday, just one day after Carrie's death on Tuesday at the age of 60. An ambulance responded to a call from the actress' Beverly Hills home at 1:02 p.m. and transported her to Cedars-Sinai Medical Center, where she later died. She was 84.
Reynolds' son, Todd Fisher, spoke with ET shortly after his mother's death, revealing that her last words to him were about how much she missed Carrie.
"She missed her daughter and wanted to very much be with her," Todd said. "She had been very strong the last several days. [There was] enormous stress on her, obviously. And this morning she said those words to me and 15 minutes later she had a stroke and virtually left."
On Thursday, Todd took to Twitter to remember his mother and sister with a heartbreaking drawing of the pair's iconic movie characters: Singin' in the Rain's Kathy Selden and Star Wars' Princess Leia, respectively.
"This is a beautiful love story to witness in my 58 years," he tweeted. "I miss them both so much. Love is everlasting."