Countries like the UK, Thailand, Australia, and Japan drive on the left, which throws off most right-hand-traffic drivers for the first day or two.
Roundabouts are usually the first hurdle, since they flow clockwise instead of counterclockwise. Go slowly, follow the car in front of you, and don't overthink it. Rental cars in these countries are typically right-hand-drive as well, so the wheel and gear shift sit on the opposite side from what you're used to.
A few habits help: have a passenger call out your lane position on quiet roads, stick a reminder note on the dash, and skip driving on day one if you're jet-lagged. Narrow country lanes with oncoming traffic tend to feel tighter than they actually are at first.
Most people find their footing within a day or two. Starting somewhere quiet before tackling city traffic makes the switch a lot less stressful.