I am a Britisher who has lived in North America for many years. Over
the course of time, I have, of course, become accustomed to American
idioms and incorporated them into my own speech. However, I'm not comfortable
with a sentence like this:
I didn't use to like her, but I have now fallen madly
in love with her.
Wouldn't this be better:
I used not to like her, but now I have fallen madly
in love with her.
Both didn't use to and used not to,
as in your examples, are correct.
Used not to is called "formal style" by
Michael Swan (a British writer) in Practical English Usage, 2e
(Oxford University Press, 1995). He lists "didn't use to"
as an informal style.
The second entry for used to in The Collins COBUILD
English Dictionary (HarperCollins, 1995) includes these words:
"If something used not to be done or used
not to be the case, it was not done in the past or was not
the case in the past. The forms did not use to and
did not used to are also found, especially in spoken
Quirk et al. (A Comprehensive Grammar of the English Language,
Longman, 1985 p.140) lists "He usen't to
smoke" and "He used not to smoke" as
preferred by many in British English," and "He didn't
use to smoke" and "He didn't used to
smoke" used by both British English and American English speakers.
Another British writer, L. B. Alexander, in Longman English Grammar
(Longman, 1988), states in this first line of his explanation that used
to may be formed without the auxiliary "do" as in
"You used not to smoke." But he adds that
didn't is more commonly used to form negatives with
used to. Alexander also states that "We can avoid
the problem of the negative by using 'never..' 'Fred never used
to be so difficult.' "
If the negative of used to is a problem, the insertion
of "never" is a way to solve it. However, since you asked,
The Grammar Exchange does not think that the negative of used
to is a problem. Either the "formal" or "British"
used not to is fine, as is the "informal"
or "American" didn't use to (or didn't