Williams revealed she fell into a "deep depression" after Destiny's Child reunited during Beyonce's Coachella set in April.
"I am just sitting here and fighting back tears," Williams told Roberts. "I look back over my journey, I look back over our journey, I'm here, we're here. I'm just thankful to be here to tell this story."
Back in July, the singer spoke about her mental health struggles on Instagram writing "depression sucks, but my faith in God and my commitment to doing the work to stay well is my way out."
In her on-air interview, Roberts began by thanking Williams and her fiance Chad Johnson for "sharing the story."
"Nobody ever wants to talk about it and it's just so important," the ABC reporter said.
Roberts asked Johnson if he was "aware" of what was happening with Williams before she sought treatment. Johnson, who's been engaged to the "Survivor" singer since March, said he had "no idea" that his bride-to-be was suffering from depression.
"The relationship just seemed to be slipping out of my hands," he said. "I could see [her] spiraling, but I had no idea that it was depression."
Williams explained she knew something was off when she began to start to feel "low again" during the beginning of the year. Williams recalled that she suffered alone and didn't want to tell anyone about what was happening, because this was her second "heavy" time battling depression.
"I was like, 'Just fight it, you've been here before,'" she said. "I'm identifying it...I just didn't do enough...so for months, I was slipping and slipping and slipping [and] before you knew it I was at the bottom of the pit looking up like, 'Am I really here again?' And I suffered by myself. I didn't want to tell anybody."
"I didn't want anyone to be like, 'Oh my gosh, here we go again. I thought you were over it,'" she added. "I thought I was practically healed of it."
Williams said that she was experiencing a "dark, heavy anger" and was questioning own existence.
Through couples counseling, Williams was able to "confront" what was making her so angry.
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The "Independent Woman" crooner went on to speak about her time in the mental health facility and ending the stigma surrounding these issues.
"When I was in the mental health facility, I didn't see anybody that looked crazy," she said. "I didn't see anybody strapped up, I didn't see anybody doing crazy behavior. And literally since then, I watch my mouth. I don't call people crazy anymore. Some people...they just need help."
Williams planned to talk about her story "one day" but was forced to speak out when she did because of the paparazzi.
"It hurt my heart because that's what keeps people from getting treatment -- somebody might find out," she said. "I had to close my curtains. I even left the treatment facility early because I was so paranoid."
"I need the sunshine [and] I could no longer have it because somebody just had to tell it," she continued." We've lost so many amazing people, whether you're in the spotlight or a loved one at home couldn't go [to treatment] because they were worried about, 'is somebody at my job going to find out?'"
Because of her depression, Williams offered Johnson out of their relationship. Fortunately, Johnson was there to support her.
"I said 'Babe, I will understand if you leave. I can go to bed at night knowing that you left because [you] were praying for a wife but not a depressed one,'" Williams said. "But he showed me sickness and in health already before marriage."