Kate celebrates her fifth wedding anniversary on April 29 - the date she married into the House of Windsor and became a future Queen in 2011.
Ingrid Seward, editor-in-chief of Majesty magazine, said the Duchess has benefited from "best friend" the Duke of Cambridge's help, but that she remains cautious about not getting it right.
"I think she's integrated extremely well. It's not easy - not a lot of guidance - but of course she has the best teacher in William. I think that's made a massive difference - having his hand on her hand," the royal writer said.
Kate Middleton was a history of art student when she met William at St Andrews University in Fife. The pair became flatmates and dated for more than eight years before finally marrying in Westminster Abbey.
Official duties beckoned - from overseas tours to jubilee celebrations - amid intense media interest. The pair also began a family of their own with the arrival of Prince George and Princess Charlotte.
Seward described the couple as a vital source of support for one another, adding: "I think she's probably his best friend and vice versa. You get pretty isolated up there in that ivory tower of royalty."