Hello, Meriem, and welcome to the Grammar Exchange.
What is the difference between:
1- I always used to do sth.
2- I was always doing sth.
The progressive form of (2) suggests continuity of the action. Instead, in (1) "used to" merely indicates a habit. Compare:
1a- When I was a boy, I always used to eat chocolate. (I was in the habit of eating chocolate.)
2a- When I was a boy, I was always eating chocolate. (I ate chocolate all the time.)