First, I would like to tell you that I truly, very sincerely admire you for being so thoughtful and generous toward your Daddy!
That shows amazing emotional maturity and makes a wonderful example to follow, thank you! ♥
As for your concern at hand, I just would like to hint a suggestion that might help. It is extremely difficult for me to explain this in English and I do hope everything makes sense..
I personally think that "letting go" is a slow process and not only is it a mental process but it is also a physical process. Sometimes the mind switches but not the body, or vice versa, and that's when it gets really hard because you can never fully reach that mental state you're seeking. The way our body reacts it tightly linked to the messages sent by our brain, a bit like a whole train is started, powered by the steam locomotive… And it takes time to slow down, or reverse and change direction. If there is nothing around your Daddy that will physically prompt that change, a part of her is likely to stay in Dominant mode and trigger a lot of struggling because she will be constantly forcing herself to be what she's not.
That's why I would suggest to create a tangible ritual for yourself and your Daddy. Something that would send the right message to her brain, so it starts releasing the right chemicals in her body so her whole being can fully engage in a different role.
This is an example, so maybe I can be a bit clearer: choose together an object that is neither hers not yours. It belongs to the both of you and represents dominance/control/caring (a card, a key, a ring, etc). Sit facing each other, your daddy holding that object. Let her give you the object and say aloud that she wants you and allows you to take control (switch mentally) then you both get up and take the other's seat (switch physically). Confirm that you want and take control by saying it aloud. Take a time to relax into the feelings it will convey and right after that, you Sparrow get to suggest an activity that will formalize your temporary control (a cuddle, a bath, a nap, or whatever best fits your unique relationship).
A ritual of candles could also be effective. Sit at the table, each with a candle placed in front of you. Your partner has a burning candle, yours is out. Your partner puts you in control by lighting your candle with hers, then putting out her own candle to symbolize the voluntary, temporary loss of control.
I really hope my English didn't fail me and that what I said makes sense and helps a little. Xxxx - Bee