As a UK driver (left) who has driven in Spain and France (right) I'd say remembering which side of the road to drive on is easy when there are other cars around, less so when it's an empty country lane. Roundabouts are tricky. Also rules around priority can be different which is hard. Then you have to remember the customs can be different, in the UK flashing your lights means "after you" whereas in Italy, it means "I'm going", or so I'm told.

I rented a car both times after going to Australia. In terms of driving on the left you adapt near instantly.

Dont worry about this and rather focus on the differences in driving laws. Australia for example tickets at 1km/h over their artificially low speed limits. #revenue

Take a vacation in the Caribbean, like the Cayman Islands. It's all right-hand driving there. Rent a car and see how easily you can adapt after a little bit of time. It's not as hard as you think.

Until you stop thinking and come out of side turning onto a completely empty road.

I'm from the UK and have driven in Europe several times, both in a local hire car and my own car.

The biggest adjustment is getting used to changing gear with a different hand. If you're driving an automatic, it's probably even easier.