I've gone through bouts of trying to adopt this habit. One thing that makes it harder is that the door handle in my car is designed to make it easy to open with the door-side hand. It is forwards on the door (near the hinged end) and opens by pulling the handle back.

If the handle was at the back of the door it would promote the habit as it would actually be easier to open the door by reaching across with the opposite hand then try to scrunch your door-side arm back to get at the handle.

I wonder where the door handles are located in the majority of cars in Holland.