Yes, a supportive navigator/keep-left reminding passenger is essential to successfully operating “on the left.”
Getting used to an enormous tractor hurtling towards you on an extremely narrow Irish “R” road, at relatively high speed, just after you’ve come around a blind curve, still takes some effort, even after it’s happened many, many times. I screamed more times than I can say this past spring, but my husband managed to keep our rental Seat economy car from becoming a hood (bonnet) ornament on a huge Ford tractor. I am still impressed by the County Cork farmers who manage to operate their tractor rigs as if they were sports cars.
Even after gaining some level of experience shifting the manual shifter (gear selector) with the left hand (and I’m left handed, BTW), there’s still the occasional grinding of a gear. Left-brain/right-brain, shifter and clutch coordination takes some skill and repetition, but at least it’s a rental.
Then there’s the matter of using the turn signal lever - unexpectedly the windshield (windscreen) wiper swipes across the glass. Oops - guess that wasn’t the proper lever, and that still takes some getting used to.
Missing scenery is a potential downside, but that’s the case in some “right” side of the road countries, too. Having a tailgater in France or Italy, and cars coming at you in your lane on a curve in Greece prevent much gawking at the scenery. Even driving the slower 80 KPH speed limit in Norway makes it tough to enjoy much scenery, when the curvy roads have a deep fjord just off to the side. In any country, pulling off to the side, when possible, will reward a driver with scenic benefits.
Occasionally walking to the wrong side of the car - left if you’re driving, right if you intend to get in on the passenger side - can still happen, even after lots of practice. It sometimes takes a second attempt to reach for the seatbelt on the correct side, since it’s the opposite of what you’re used to.
Parallel parking on the left curb - surprisingly easy. It’s possible to be an ambidextrous parker.