The 62-year-old fashion designer was forced to take over as the head designer at the prestigious label when her late brother Gianni was shot dead outside of his Miami Beach mansion in July 1997, and the blonde businesswoman felt "the eyes of the whole world" on her to continue the brand's successful legacy after losing her sibling.
Speaking to the Guardian Online, the creative mastermind said: "When my brother was murdered, I had the eyes of the whole world on me and 99 per cent of them thought I wasn't going to make it.
"For the first five years, I was lost," she says. "I made a lot of mistakes."
And the fashion mogul has revealed she also doubted her ability, as she believed her brother was "the king".
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She added: "And maybe I thought the same, at first. My brother was the king, and my whole world had crashed around me.
"I was his doll and his best friend. He dressed me up in cool clothes, took me out to discos and clubs from when I was 11. I loved it. It was the best time of my life."
Donatella has admitted if she was to relive that time in her life she would tell her younger self to "be strong" and to follow her own ideas instead of trying to be like Gianni because she has realised she was her brother's muse and advised him on his creations.
She said: "I would say to myself: be strong, and stay true to yourself. Oh, there are a lot of things I would love to say! But most of all, follow your own instincts, and don't try to be Gianni.
"I had been listening to everyone else, and then I realised, who was the person my brother listened to? Me. I worked with him every day. I was much more than a muse. It was a dialogue between us. We discussed everything."
Meanwhile, the mogul - who has 31-year-old daughter Allegra, and 27-year-old son Daniel - has claimed she wasn't born "fantastically beautiful" or tall, which is why she wears high heeled shoes "all the time".
She said: "I was not born fantastically beautiful, but I always wanted to be impressive. So I bleach my hair blond, I wear high heels. I am 5ft 2in - me and Bruno Mars, the same - so I wear high heels all the time, to be tall."