Cameron Diaz was ''so scared'' before the release of her book 'The Body Book: Feed, Move, Understand and Love Your Amazing Body'.

The 44-year-old actress admitted she felt debilitating fear prior to sharing the self-improvement tome with the world in 2013 and the stress made her ill.

She said: ''Right before the book came out, I was on my back for a week. My back went out. I couldn't breathe.

''I've never had that kind of thing where I was just literally debilitated. I realized I was just so scared that I was going to put this thing into the world and everybody's going to see me, and what if they don't like me? What if they think I'm crazy or I'm stupid? I was like, oh God, this is so painful. It was revealing about who I was. Movies are like totally fake. It's not me at all. It is, but they're characters. This book was all me. There was no hiding. I was so open and so raw.''

The book becoming a New York Times bestseller but Cameron says she was most proud of the fact she was able to spread her message to other women

She added: ''The book isn't the numbers that got sold. It was that I had an intention, and the intention was to get that information out to as many women as I could.

'The Mask' star went on to write her 2016 book 'The Longevity Book: the Science of Aging, the Biology of Strength, and the Privilege of Time' and spoke about what success means to her success at sister in-law Nicole Richie's Pearl xChange at The London in West Hollywood on the weekend.

According to People, Cameron - who is married to Benji Madden - said: ''You have to forget about what success is to everybody else, and you have to know what success is to you. You have to know that it's not about how much money you make, or what car you drive, or what bag you buy, or how many likes you get. It's all about your intention of what you want to accomplish on a personal level.''