I had more trouble after coming back from a few weeks in England than while driving there.
Overseas, I always had my guard up. Back home, I’d occasionally find myself on the wrong side for up to 6 moths after returning. The accent took a couple months to lose too.
I think we let our guard down when stuff goes back to “normal”, yet learned habits, even short term, can die hard.
I drove a couple rental cars in New Zealand for a total of 17 days. I just remembered to keep me nearer to the center line than my wife and I was fine. I did catch myself a couple times looking left for traffic and almost pulling out in front of someone coming from my right. My biggest issue, as I have stated a couple other times here, was signaling my turns with the windshield wipers. The wiper switch was on the left, the turn signals were on the right.
I do remember a story about a woman driving on the wrong side of the road in the US that killed a motorcyclist. There was a lot of outrage in MC circles because she’d successfully fled the US and wasn’t likely to face justice. This would have been late '90s or early '00s.
I have lived for decades in India, where all traffic is keep-left, and drive here in the US in keep-right mode. Never faced a difficulty so far. Neither have others in my community, who have driven for long in India, too. I suppose most people just adjust successfully and drive mindfully. In 2017, just 2.3% of fatalities were due to wrong-side driving.
And that was the crux of the problem for me while I spent a week working in Cambridge.
Every day the drive to and from work was tense, but things worked out fine.
Until Wednesday afternoon when it started snowing. That covered all of the markings on the pavement. I came face to face with a kind bus driver who must have understood I was a dumb yank and didn’t even honk.
This is very true.
When I lived in Japan, I had no trouble driving on the left. The self-preservation instinct has a very clarifying influence when oncoming traffic is coming right at you.
But - there were several times I almost got hit crossing the street because I was habituated to check the wrong lane.
I also did stuff like getting out my keys, seating myself in the passenger’s side, and wondering momentarily who stole my steering wheel.
Personally, I did not find ‘driving on the wrong side’ to be the significant challenge when driving in Spain (after 30+ years driving in the UK).
The challenge was:
[ul]
[li]Remembering to look the right way at junctions - i.e. give way to traffic from the left at roundabouts[/li][li]Noticing signs and signals such as traffic lights[/li][/ul]
Staying on the right side was indeed my significant fear in advance, but in practice, that part was really easy - it was the other things above.
Going to the wrong side of the car is universal, I think. I never actually got in the wrong side when I was vacationing in NZ, but I kept going there and sometimes opening the door before remembering. The pile of stuff I kept in the passenger seat was a reminder.
I didn’t have any problems keeping to the correct sidfe of the road. My main issue was that on the second time I went there, I forgot to specify I wanted an automatic transmission and the default in NZ is standard. Well, I’d been driving a manual transmission since forever in the States, but the wrong side of my brain is trained to shift. So I kept making all kinds of shifting errors. After about three days, I adapted.
My wife’s job as passenger during our down under vacations was to remind me when it looked like I was headed for the wrong side of the road- which she did a few times. 
Most emphatically.
I spent a few days in Delhi two years ago. I don’t imagine traffic is too dangerous when it’s not even moving. But then, we left the driving to the experts.
When it begins moving, that’s when the fun begins! You did right by not driving yourself. It takes balls of steel to drive in India, especially the metros.
nm
My mom returned to Montreal after a few years in India/Bangladesh and I recall being in the car with her once when she was casually (and incorrectly) on the left. I pointed this out to her and she adapted. I retrospect, we were in a neighborhood that I doubt she had visited very often before her departure. On more familiar streets, she had less difficulty and rapidly adjusted.