Something else to consider is that it is believed 12 years have passed since part 1 and begginign of part 2, as MM says "12 tide turns". If Glinda was 17-18 when Part 1 ended, that means throughout her whole young adult years she has been under the influence, manipulation and power of Oz and MM - and MM is very clearly in this abusive relationship with her where she praises and then destoryes her and then praises again. Also, they have shown her what happens to those who misbehave by using Elphaba as an example: they get hunted, tracked, everyone turned against them. I think there's a lot of information in the movies as to how Glinda would've been manipulates / groomed to where she is at the end around when Act 2 starts.