(ABC) -- Kristen Bell has battled depression for more than 15 years.
The "Frozen" actress opened up about her struggles with the mood disorder in a candid new essay for Motto, hoping to lessen the stigma around mental illness.
"I didn't speak publicly about my struggles with mental health for the first 15 years of my career," the star of "Bad Moms" wrote. "But now I'm at a point where I don't believe anything should be taboo. So here I am, talking to you about what I've experienced."
While a student at New York University, she said, "I felt plagued with a negative attitude and a sense that I was permanently in the shade."
"I'm normally such a bubbly, positive person," she added, "and all of a sudden I stopped feeling like myself."
Instead, Bell said her depression made her feel "a complete and utter sense of isolation and loneliness." And, despite having friends and work she loved, the actress said, "I felt worthless, like I had nothing to offer, like I was a failure."
Bell said her mother sat her down when she was 18 and spoke with her about the possibility of mental illness. She encouraged her to seek help if she ever needed it.
Now Bell said she wants to be "candid about this so people in a similar situation can realize that they are not worthless and that they do have something to offer."
"There’s nothing weak about struggling with mental illness," the star said, noting that 20 percent of American adults deal with some form of mental illness. "You’re just having a harder time living in your brain than other people."
"Depression is a problem that actually has so many solutions," she added. "Let’s work together to find those solutions for each other and cast some light on a dark situation."