File photo: Britain's Prince William and Kate, the Duchess of Cambridge with their children Prince George and Princess Charlotte arrive at the Christmas Day service at St Mark's Church in Englefield. Picture: AP

She has all the privileges most parents can only dream of. But the Duchess of Cambridge admitted that even for her motherhood has its difficulties.

Kate said that despite the help of a full-time nanny and a team of household staff, she still finds coping with Prince George, three, and 22-month-old Princess Charlotte an occasional struggle.

In her most personal speech yet, at an event to highlight the issue of maternal mental health, she said some of her experiences as a mother had led her to feel "unconfident and ignorant".

She said: "Becoming a mother has been such a rewarding and wonderful experience. However, at times it has also been a huge challenge – even for me who has support at home that most mothers do not. Nothing can really prepare you for you the sheer overwhelming experience. Your fundamental identity changes overnight.

"And yet there is no rule book, no right or wrong – you just have to make it up and do the very best you can."

Kate, 34, who wore a striking Sixties-style dress by Eponine, warned that for the two in ten women who suffer mental health issues during and after pregnancy, the pressure can be even greater. Post-natal depression can "cloud their moments of joy with a real sense of darkness and isolation" but leave them unwilling to seek help for fear of not being seen as coping.

"Some of this fear is about the pressure to be a perfect parent; pretending we’re all coping perfectly and loving every minute of it," she said. "It’s okay not to find it easy. Asking for help should not be seen as a sign of weakness."

The event, organised by the Best Beginnings charity, was the first time she has spoken in such depth about her experiences. It is known that George did not sleep well as a baby and that she and William found parenthood tough at first.