USED TO / SIMPLE PAST

"Used to" refers to things that happened at an earlier stage  of  one's life and are now finished: there is an idea that circumstances have changed. It is not used simply  to say what happened at a past time, or how long it took, or how many times it happened.

Examples:

I studied very hard last month ( NOT  I used to work very hard last month)

I  lived in Hastings for three years (NOT I used to live... because we are saying  how long ( for three years)

Mary went to London three times last year (NOT  Mary used to go...because we are saying how many times ( three times last year)