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"Looking at the bigger picture, going back to stay with the nanny is a short-term solution. I should think about Anna as a permanent feature in my life with my boyfriend should we get married. Like it or not, I will be her stepmother, and I can't keep offloading her to someone else because she can't take care of herself and I refuse to play caretaker or teacher. In my mind, I can hardly take care of myself."
I know of distanced arrangements like this in Asia, particularly among the wealthy, but your writing fluency and sudden angst after so many years suggests you are not Asian. Why is the nanny a short-term solution? She's been the parent for SIX YEARS. The entire time the child has been back in your country. Why, after six years of not offloading, but standing by why her father offloaded her, do you suddenly think that a wedding ring obliges you to think differently? I think you need, PDQ, a conversation with Dad. "I can't and won't play parent, what impact does that have on our marriage intentions? ...Are you intending her to stay on with Nanny?...I think she should be evaluated/given therapy etc.". If this letter is true (and I rarely doubt them but this is so callous it's a little unbelievable), the money seems to have divorced the entire situation from sanity if not reality. You're going to have to (a) settle for more of the same living arrangements (she goes to the Nanny's where she is at least welcome), or you can (b) create this entire drama, that Dad doesn't appear to feel, around your sudden change in obligation after a wedding.
And at 29 why on earth can't you take care of yourself? I'd suggest you hang on to Daddy's financial cushion if you're so helpless yourself.