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A friend had to teach her 9 year old stepson, who lived in another country with a super-coddling mother (the child reported to his teacher first day of school that his name was "mummies little darling"!): how to dress himself, yes really; how to handle cutlery; to chew with his mouth closed; to say please, ever etc. And this child had a doting mother, but one who did absolutely everything for him. It's a simple equation - no expectations and having everything done = no learning. By the end of a 2 month visit the boy could handle himself pretty much as you'd expect of a 9 year old. Nannies often are very indulgent. But this Nanny's love may be the linchpin that has held this girl together. Mentally ill mother, abandonment at 10 years old, they are very tough circumstances to deal with and she isn't acting out to her credit.
I do think it brave of the LW to be honest about her feelings - if you're going to be PC even with yourself you'll only tie yourself up in knots.