In slightly older English (say Jane Austen) you'll find 'use' = 'be accustomed to' more alive in the construction: 'I do not use to do this'. It is now frozen, only the past tense is used, there is a problem with the negative, and the pronunciation has changed.

I would like a convenient contraction like 'isn't' or 'gonna' or 'hafta' for the actual pronunciation, but nothing lends itself: 'ustu'/'usta' looks glaringly un-English or fails to show the sound. I believe (offhand) that it's never pronounced like the normal verb 'used' with a /zd/.

As for the negative, is it "didn't ustu" or "ust not to"? Even in my own speech I can never decide.