Congrats on progress! You'll be amazed that it will become second nature.
Going back and forth from rear view mirror to looking over my shoulder will mess me up momentarily every once in a while which is frustrating. At this point, my brain makes the shift if I don't think about it and just do it.
Best thing for me learning was as a teenager I was put on a tractor pulling a utility trailer and forced to back through two lines of shovels with about 6" clearance on either side. A tractor gave full visibility so that was out of the equation - probably part of what delayed my backing ability with mirrors. Anyway, I was supervised by one of my school teachers who was a close friend of my dad. As I got better, he started yelling at me, distracting me, whatever he could do to make me nervous. Once I could back it while nervous, he said, "You're good." I'm sure someone today would object to his technique but it was effective at teaching me to execute a skill under stress.
For anyone learning to back-up, I recommend lots of practice with cones in a parking lot and making it tighter than you expect in a normal campground.