"It happened to me on the night before the first ever Spice Girls live performance," she said. "We were in Istanbul, we did two shows over there, and we'd never done a full-length concert before, so obviously we’d rehearsed for weeks ahead, costume fittings, makeup here, everything was leading towards the pinnacle of everything I’d ever wanted to do and ever wanted to be."
"What drives me is being on stage, being a performer, so here we were the eve of the first ever Spice Girls show, so I treat myself to a massage in the hotel," she continued, adding, "And what happened to me I kind of buried immediately because there was other things to focus on. I didn’t want to make a fuss, but also I didn't have time to deal with it."
"Because I didn't deal with it at the time, I realize that I allowed that to be buried for years and years and years," Mel said.
She later added that the incident went down while she "was in an environment where you take your clothes off with this professional person," sharing that it "has affected me. But I buried it. Lots of people do."
After the memory was locked away for so long, the "Dancing with the Stars" alum said the incident came back to her in a dream, prompting her to consider including it in her memoir.
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"When I was writing the book it came to me in a dream, or I kind of woke up," she said, "and it was in my mind and it was like 'Oh my gosh, I haven't even thought about having that in the book.'"
She continued, "Then of course I had to think 'Do I want to reveal this?', and I just thought 'I think it's really important for me to say it and to finally deal with it and process it' -- and for other people."
"I suppose in a version of sexual assault it's a mild version but I felt violated. I felt very vulnerable. I felt embarrassed, and then I felt unsure 'have I got this right, what's going on?" she added.
Meanwhile, during an appearance on the "How to Fail with Elizabeth Day" podcast earlier this week, the "Wannabe" singer -- who has previously spoken about her battles with depression and an eating disorder -- opened up about her struggles with mental health.
"Even now, in 2022, I live with depression. You know, it's there. I've learned very much how to deal with it and cope with it and keep it at bay, but sometimes it can get the better of me," she said. "So I think it's really important to learn what works for you. I think everybody can just have different little tools in the kit to get them through."
Mel revealed that she's previously felt suicidal, calling it an "awful" experience.
"I like to think I've never felt suicidal, but I have wished to not wake up which is awful -- which is an awful, awful place to be in," she shared. "Sometimes it felt like my spark had gone out. But most of the time it's there. There's a little flicker even in my darkest moments and it's pulled me through."
"I would say it's so important to speak," she continued. "I used to hate...'I don't want my mum to worry, I don't want my friends to feel like 'oh god, here she is again', and not want to pick up the phone."
Mel added, "I'm a warrior. I was struggling and in my eyes I failed. I failed myself, I failed the public, but I got through it. I got through the other side. My story is that I did succumb to these things and I felt ashamed for that, but the thing that I feel very proud of is that I overcame all those things."
Despite how fame has affected her mental health, Mel said she ultimately, "wouldn't change it."
"When I talk about the really tough times I think 'I wish I'd done that differently,'" she admitted. "But if I had to do it all again the same way I would because I love my life. I've achieved my childhood ambition and dream and I'm still doing it.'
"The Sporty One: My Life as a Spice Girl" will be released on September 27. The UK edition, titled, "Who I Am: My Story," is out now.
The National Sexual Assault Telephone Hotline -- 800.656.HOPE (4673) -- provides free, 24/7 support for those in need.
If you or someone you know is struggling with an eating disorder, please contact the National Eating Disorders Association (NEDA) at 1-800-931-2237 or go to NationalEatingDisorders.org.
If you or someone you know is struggling with depression or has had thoughts of harming themselves or taking their own life, get help. The National Suicide Prevention Lifeline (1-800-273-8255) provides 24/7, free, confidential support for people in distress.