I advocate "used not to speak". This sounds better to me than "used to not speak". "didn't use to speak" may be just about tolerable in speech, but it looks horrible written down, in my opinion.
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AnonymousWhat about that "often" though?The presence or absence of "often" does not affect my previous answers.
AnonymousI wrote "used to not". Everyone thought that I just messed up the order of "not" and "to",They were correct. You did. See and the other link there.