04/20/2011 at 11:00 AM EDT
When Catherine Zeta-Jones
received a recent diagnosis of bipolar II disorder
, a mental illness marked by prolonged periods of depression alternating with episodes of mild mania, she didn't hesitate to step forward with the news.
"This is a disorder that affects millions of people and I am one of them," the actress, 41, tells PEOPLE in an exclusive statement in this week's cover story. "If my revelation of having bipolar II
has encouraged one person to seek help, then it is worth it. There is no need to suffer silently and there is no shame in seeking help."
In early April, the Oscar-winning wife of actor Michael Douglas, 66, and mom to son Dylan, 10, and daughter Carys, 8, checked in to Silver Hill Hospital, a mental-health facility in New Canaan, Conn.
Shaken by Douglas's battle with throat cancer
, Zeta-Jones was privately struggling, a friend says. As Douglas recovered, Zeta-Jones didn't – and with her new movie, the romantic comedy Playing the Field,
slated to start shooting in Louisiana, the actress realized she needed help in the face of mounting depression.
"The simple things would just seem overwhelming, like going out to dinner," says the close friend. "There was just a little piece of her chipped away. It was hard to watch because I knew how hard she was trying."
For much more on this story, including details of Zeta-Jones's private struggle and how she decided to get help, pick up this week's issue of PEOPLE, on newsstands Friday