Wow, it seems some people just adjust pretty quickly, while others it takes a while. If you don't mind my saying, I also notice that with those who don't adjust as well, it's usually out of fear, or nervousness. Maybe it would help to just relax about driving and take a more "natural" approach? I wouldn't know because I haven't driven in the UK, just throwin' out an observation.
I think part of the fear is trying to adjust to the different sizes in roads on top of driving on the other side of the road - I know that made me very nervous when I was learning to drive and I was born and raised in the UK (it took me a year of weekly lessons and two failed tests before I finally passed, and then another 2 years to feel fully confident on the road and comfortable driving in areas I wasn't familiar with)!
The US is full of big, wide, straight roads, often with 4-6 lanes and plenty of space, whereas the UK roads are much smaller and less straight and it can be very daunting trying to drive along a
twisty, windy country road that only has enough space for one car to fit in... and then there's a big tractor coming your way and you have no idea how it's going to get past you without crashing right into your tiny little car
! Or trying to drive down a tiny street with
cars parked on both sides of the road so there is only enough room for one car to fit through and there are cars coming towards you!
Plus, there's the parking spaces which seem to be about half the width of US parking spaces, and the roundabouts, and the unfamiliar road signs and rules of the road. There's also pedestrians and bicycles to worry about, and people driving faster on the motorway than they usually do in the US (you see a lot of people going 90-100 mph, even though they're not supposed to).
Coming from the US (where I am currently living), I can understand why people would be nervous and afraid to drive on the UK roads. I have relatives who have lived in the US for 30 years and even though they learned to drive in the UK originally, they have spent so much time driving on large US roads that they get scared by the small country roads when they come back home to visit.