Carrie Fisher (left) and Debbie Reynolds
02/28/2013 at 07:00 AM EST
Carrie Fisher was briefly hospitalized last week for her bipolar disorder after a bizarre performance aboard a cruise ship, but her mother Debbie Reynolds says she'll be fine.
"She's had manic depression bipolar since she was 13. It's an illness, and she's doing much better," Reynolds told PEOPLE exclusively Wednesday night. "I'm very proud of her, and she's doing exceptionally well. She'll be just fine, just great, and continue her writing as she always does."
Fisher, 56, who has spoken openly about her mental illness
, performed on the Holland American cruise liner Eurodam in the Caribbean. The actress and author gave a rambling performance, leaving many in the audience wondering what was wrong.
"She clearly had trouble remembering things," says Chris Smith, a guest on the cruise. "She tried to tell some stories about her parents and Hollywood, but was having a hard time."
After a video of the performance went viral, her publicist released the following statement: "There was a medical incident related to Carrie Fisher's bipolar disorder. She went to the hospital briefly to adjust her medication and is feeling much better now."
Reynolds, 80, who spoke Wednesday to a sold-out audience attending the Rancho Mirage Lecture Series at Eisenhower Medical Center in Rancho Mirage, Calif., talked about the effects of mental illness.
"[It's] really dreadful, and you are so alone because you're criticized, and people think you're doing it on purpose, and that you're misbehaving or having a spell because you want attention," she says. "It's not true. It's extremely difficult for everyone to deal with."