She has that scribbled in her excercise book at the beginning of The Prom. The characterisation in S3 is still very episodic compared with the later seasons.The Zeppo being the most egregious example but Dopplegangland does it as well, lays out a previously not particularily obvious character flaw so it can be overcome by the end of the episode. For Buffy The Prom is all about having that one good High School moment to remeber (not to remain in eternally). She gets infantalised by both Angel and Joyce making decisons for her and actually takes it quite maturely (she is too forgiving of people doing this, she forgives Giles very easily in Helpless, maybe it's just happened so often she's innured to it). That changes though by S7. Maybe even by S6 - she doesn't really forgive the Scoobies for ripping her out of heaven, just bottles it up until Normal Again.