She resolved to take action when she stood on the scales and realised she weighed almost the same as when pregnant with her first daughter, Princess Beatrice, now 25.
"That's what really frightened me," she says. "Beatrice was an 8lb 8oz baby and I was only 6lb off my full-blown pregnancy weight. I decided to make a change."
It was in the swish Swiss ski resort of Verbier that Sarah Ferguson, 54, underwent her stunning physical and mental transformation.
"I used to be so angry," she exclusively tells HELLO!. "I believed I was never going to lose the weight, that I had lost control. I couldn't fit into any of my clothes. I was just drowning in eating, drowning in food."
Sarah, a former Weight Watchers ambassador, cut out sugar and stuck to a straightforward exercise regime to achieve her new slimline figure.
"It took courage to go out in public in all the years I have been overweight. Every time there was a 'Duchess of Pork' or 'Fat Frumpy Fergie' headline, they didn't have any idea they were reaching to the depths of my soul," she says.
HELLO! can reveal she has been appointed an ambassador for the Institute of Global Health Innovation at Imperial College London.
SEE THE FULL EXCLUSIVE ARTICLE IN HELLO! MAGAZINE, OUT NOW