British royal Duchess of Cambridge's sister Pippa Middleton has opened up about her life in the spotlight, insisting she's still struggling to make sense of her new-found fame.

Middleton caused a media frenzy during her sibling's wedding to Prince William in 2011 thanks to her shapely posterior - and since then, she has been hounded by the paparazzi around the globe.

Now the notoriously private 29-year-old has confessed in her new party planning book, Celebrate: A Year of Festivities for Families and Friends she's having a hard time dealing with the constant attention.

"It's a bit startling to achieve global recognition before the age of 30 on account of your sister, your brother-in-law and your bottom," she writes.

"One day I might be able to make sense of this. In the meantime I think it's fair to say that it has its upside and its downside... I can assure you that it feels even stranger to me than it probably does to you to have seen so much written about me when I have done so little to paint a picture of myself."