By Andrea Billups

12/21/2012 at 11:15 AM EST

Kelly Osbourne

Nicky Johnston/Courtesy of Cosmopolitan Body

Kelly Osbourne credits her recent weight loss and new career as a style expert to one (pretty simple) thing.

“People think I lost weight and that’s what made me happier. That’s not true: I had to learn to love myself first,” Osbourne tells Cosmopolitan Body, a spinoff of British Cosmopolitan magazine, in its latest issue, available Dec. 27.

“Losing weight was just one benefit of putting the hard work in and sorting myself out on the inside first through therapy,” Osbourne, 28, says of her personal transformation, which involved losing more than 60 pounds. “That was one of the scariest times of my life. I swear I’ve never felt more naked, because I had to actually be me and couldn’t mask it.”

Happily in love with boyfriend Matthew Mosshart – and now comfortable wearing a bikini – the lavender-haired Osbourne, co-host of E!’s Fashion Police, says there’s no simple trick to losing weight. She’s just careful about her food – enjoying pizza and chocolate cake in moderation. But she also exercises every day, something she says has become addictive in a good way.

“When I started exercising I couldn’t even do one sit-up,” Osbourne said. “By the end of the first week I could do three, and that was a big deal. Now I work out every day and I just don’t feel good if I miss it. It’s amazing how quickly your fitness builds, but you have to really want it.”

Her Cinderella moment came at last year’s Emmy Awards when she zipped up easily into a Zac Posen couture dress the designer shipped specifically to her.

“I’ll never be the kind of person who thinks, ‘I’m so hot.’ I don’t want to be,” she said.

“But I learned to respect and love myself – something I didn’t think I would ever be capable of,” continued Osbourne. “I put on my dress, and when I looked at myself in the mirror I cried. Never in my life did I think that Zac Posen would fly in a dress for me that he could have given to any nominee. It fit me perfectly!”