Countries like the UK, Japan, Australia, and South Africa drive on the left, which takes most right-hand-traffic drivers a day or two to adjust to.
Roundabouts are usually the first hurdle, since they flow clockwise instead of counterclockwise. Go slowly, follow the car ahead 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.