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.
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)