The model and actress told The Guardian that she had been encouraged to tone down her personality when working on TV and in cinema.
Brook said: "I'm quite loud. When I was in film and TV, people were always saying, 'Oh Kelly, make it smaller, make it smaller'.
"I think TV is all about not turning off the public, it's about not being too sexy, not being too much of anything really."
She added: "I think the more mediocre you are the better you do because people need to think you're their friend, they don't want to be threatened by you, you've got to be warm, you've got to be not too smart, not too pretty, not too anything."
Of her short-lived stint as a host on The Big Breakfast in 1999, Brook continued: "I was 18, I've got big boobs, it was my first TV job and people literally sat and scrutinised everything I did and wrote down every mistake.
"At that age it's hard to deal with. Maybe it was because people thought I hadn't earned it, which I completely respect and understand, but it still was a bit harsh."
Regarding her sacking as a judge on Britain's Got Talent, she added: "It upset me a bit but it didn't really change my life. I find it really hard to work in TV in this country.
"It's never worked out for me. There's a lot of jealousy, a lot of egos, a lot of backstabbing, it's not a very nice environment. That's what I've found."