My take on M’s views about being royal is not that she was ill prepared or supported.
I think she was arrogant in thinking she would have total agency over a lifestyle that was (lavishly) funded in recognition of the service and constraints the working royals are expected to adhere to.
That she would get to pick the causes and events and the schedule. The freebies, jewellery and grace and favour residences.
She presumed she’d be her own CEO when in fact she was just a pretty mid ranking “employee” of The Firm whose influence was negligible and where the scope to throw away long standing tradition and rules was none existent.
As soon as she realised that “M doesn’t get what M wants” she started planning her exit.