I wasn't used to driving on the left.-- I hadn't done it very often and I was uncomfortable doing it. I didn't use to drive on the left.- I drive on the left now; I didn't drive on the left before.
I was used to driving on the left. I used to drive on the left. These carry the same relative meanings as these (above): I wasn't used to driving on the left. I didn't use to drive on the left.