I really like this change from the manga. Not only does Ami being brainwashed build the dramatic tension and break your heart, it goes well with the arc Ami’s going through.
Ami is excited about being a role model right now. She sees how Usagi looks up to her and admires her dream to become a doctor and how hard she works. For the first time, she has the feeling of being able to inspire someone and promote admiration. She feels Usagi’s love and support. She’s able to see who she is as a good thing, and now she wants to inspire other kids to work to their fullest potential and follow dreams like being a doctor. Usagi gave her to confidence to see herself as someone who can do that.
But she gets so swept up in it, the teacher takes advantage of her feelings and literally forces her to forget that Usagi already sees her as amazing as she is and keeps her so captivated by the idea of being an inspiration she can’t even spend time with the person she’s inspiring. And that’s wrong.
She can’t just drive herself so hard that she forgets about having a personal life. She’s great as she is right now, and she doesn’t need to push herself so hard that a role model is all she is. She needs to take the time to feel Usagi’s love and support, to spend time with people who love her and relax. Usagi doesn’t just love her because she’s a role model. She loves her as a friend and person.
The brainwashing, in many ways, kinda seems like that played out as a metaphor. It works very well.