...really, if Kanna is consistently living with humans — going to school with them, participating in all the same events and rites of passage as they do — it really wouldn't be odd for her self-image to grow as quickly as they do, just from experiencing life at the same speed and to the same degree.
( There is certainly a degree of voluntary shapeshifting to all of the dragons' human forms, though I don't think they've got complete control over how they look.)