Welcome to the forum.
You have two completely different verbs here. The 'rule/exception' about the negative is pointless.
1, BE used to (with a meaning similar to BE accustomed to). Here 'to' is a preposition, like the underlined words in BE interested in, RELY on, LOOK forward to.
I am used to getting up early
I am not used to getting up early.
2. Used to. This modal-like expression (used for a past habit/state that is not true of the present) is followed by an infinitive.
I used to enjoy being on my own.
I didn't use to enjoy being on my own.