Gillberry:
As soon as the trailer starts turning you need to be getting your steering lock off and straightening the cab to the trailer otherwise the trailer will keep turning .
Put your head out of the window until the cab is straight with the trailer.
Your right the mirror gives a different view which is why head out the window is much better until you are happy to use the mirror.
Reversing is something the you can do in one or take multiple shunts but the main thing is if in doubt shunt as it’s better not to hit anything.
As others have said the fact you haven’t given up is good but don’t worry one day it will click and you will find you reverse in one most times Isay most as even those good at reversing still need shunts especially on the easier reverses as the harder it is the more you concentrate.
Practice in your yard when you can as the more you do it the easier it is to understand how the trailer moves .
I wasn’t the OP - but anyway you make a very valid point the harder the task the more you concentrate and I’ve found one performs better. Its simply practice and practice BUT for me the big difference is having one’s own truck.