Sayuri was determined to change.
She had to.
It wasn't just to pass the test, but it was also to improve her character. If she was to be the next CEO, how could she have such a weakness? The company may crash because of her and that would make those people happy.
Those who wanted to put her down. Those that looked down on her. Those that were waiting for her to fail.
Knowing your weakness was the first step in overcoming it. Taking that first step, however, required a lot of effort. Changing your mindset and manner of thinking isn't easy. It's not like one was changing clothes, but changing a basic nature of yours that had gotten a firm grip.
If a person was lazy, and he knows he is lazy, does it mean that he would suddenly be able to become hardworking?
No.
It would take supreme effort to push himself to work hard when his entire being was saying 'sleep' or 'another day'.
It was the same for Sayuri.
