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

I'm with Faldage and (probably) the rest of the Norte Americanos: used not to doesn't sound right: around here, the construction is always some form of "yoostu/yoosta," whether negative or positive. Yoostu/a is not split. I yoosta go to school, but now yoose guys are my educators.