I was used to doing

Hello,

Suppose you start with the sentence

- I'm having trouble adjusting to driving in London becasue I'm used to driving on the right.

Now you want to refer to it as a past event. How should you put it?

- I was having trouble adjusting to driving in London because

a) I was used to driving on the right
b) I used to drive on the right

I feel like a) should be correct, as b) seems to have a different meaning from the one intended. On the other hand, I have rarely encountered the past form of "I'm used to doing". I'm not sure if it is used, or even grammatical.

Can you please help me?

Thank you
H.