The hardest thing for me is if I have know you as one name and now I’ve got to use a new name. This has nothing to do with gender or politics however. It’s just how my brain stores things. My sister uses a different first name in adulthood than when we were kids, and I never have been able to adapt.

That's interesting. I'm certainly not young myself (approaching 47), and while I had no problem with remembering the new name for my high school friend who had transitioned in the years between us knowing each other then and getting back in touch, I have so much trouble remembering to call my own (cisgender) daughter by the shortened version of her full name, which she'd prefer. Maybe because my friend also changed her gender and, obviously, her look so it's easier to remember when I talk to her?