"I've dealt with depression," the singer and actress, 32, tells the Associated Press. "I had to choose to get out of bed and do whatever I needed to do to be happy."
Williams first suffered from the mental health disorder as a teen, but managed to recover without the use of medication thanks to therapy, exercise and positive thinking.
"We're taught, 'Just go to church and pray about it. The Lord is going to heal you,'" Williams explains, adding that she's opening up about her experience to encourage others to seek assistance.
"Well, in the meantime, I believe God-gifted people, physicians, doctors, therapists – that's your healing. Take advantage of it. Go see a professional so that they can assess you. It's okay if you're going through something. Depression is not okay, but it is okay to go get help."
With the dark cloud lifted, Williams begins 2013 with a lot on her plate. Later this month, she begins starring in a touring production of Broadway musical FELA! – the same day Destiny's Child is set to release their record, Love Songs.
"Stacking those harmonies on top [of] each other gave me goose bumps," she says of recording "Nuclear," the sole new track on the album that Williams, Kelly Rowland and Beyoncé Knowles recorded in Los Angeles shortly before the holidays.
"We were like, 'We still sound good together.' Duh! The bond will never die. We're always going to be close. We're always going to work together."
The trio will reunite at the Super Bowl Feb. 3, when they are expected to join Knowles onstage during her halftime performance.
"It's amazing that people are still fascinated by the connection," Williams adds. "It's been about eight years since we released original material and people still ask me, 'Do you all talk?' We could take a picture together today and then tomorrow, people would say, 'Do you all still talk?'"