If you've only ever driven on the right, switching to left-hand traffic in places like the UK, Japan, Australia, or South Africa takes a short adjustment period.
The biggest mental shift is roundabouts, which flow clockwise in left-hand-traffic countries. Take them slowly at first, and watch the car ahead of you rather than relying purely on instinct. Many rental cars in these countries are also right-hand-drive, meaning the steering wheel and gear shift are mirrored from what you're used to.
A few habits help: keep a passenger call out lane position on unfamiliar roads, place a sticky note on the dashboard as a reminder, and avoid driving on your first day if you're jet-lagged. Give yourself extra time on narrow country roads, where oncoming traffic can feel closer than expected at first.
Most travelers adjust within a day or two. Starting in a quieter area before tackling city driving is a good way to build confidence.