"[She thought] you start in a good place and there is this, no matter what, there is a honeymoon period that comes with marrying into the royal family that allows you to falter at times, or figure something out, or whatever it is," he added.
"I think that that was one of the key things, and I think that towards the end she felt she wasn't given that by the press or members of the royal family, by aides or courtiers within the households."
The royal writer went on to say Meghan wasn't the only one who was naive about how easily she'd adapt to royal life, saying the royal family also had "unrealistic expectations" about how easily Meghan would be able to adapt to her new life.
He added: "I think everyone somehow had this level of expectation from her from the get-go that was perhaps somewhat unrealistic, and perhaps she herself had some unrealistic expectations from the role too."
Victoria agreed, and went on to note that while everyone in the royal family has a "honeymoon period", Meghan's was "incredibly short".
After announcing their plans to step back from their senior royal roles, Harry and Meghan are currently staying in Canada with their son Archie.
Last week, after crisis talks, the Queen announced that she hoped "that today's agreement allows them to start building a happy and peaceful new life."
She thanked the couple for all their "dedicated work across this country, the Commonwealth and beyond".
While Buckingham Palace also released a statement, confirming "The Sussexes will not use their HRH titles as they are no longer working members of the Royal Family."
They will also repay Sovereign Grant expenditure for the refurbishment of Frogmore Cottage, which will remain their UK family home.