In chapters 29 to 38 of Cinder, Cinder begins to realize not only that she needs to gain independence from Adri but also that she's ultimately the only person who can turn the tide of evil that's sweeping over the world. She spends the last quarter of the book becoming an independent and active person who might one day be able to fulfill her destiny.
When Cinder comes home and finds that Adri has destroyed Iko for scrap, she's finally ready to leave. She wants to put the Commonwealth, her family, and Prince Kai behind her once and for all. However, she finds out that Queen Levana is going to take over the Eastern Commonwealth after she marries Kai; this danger is enough to make her abandon her long-desired plan to leave and instead plot to let Kai know the truth and stop the Lunar invasion.
Cinder becomes more independent and active at this point because she has a goal that isn't just about removing herself from her abusive living situation. Instead, she wants to do something proactive by working to save not just herself but everyone in the Eastern Commonwealth from Queen Levana. Ultimately, however, her attempts to warn Kai only lead to her being discovered as a Lunar by Queen Levana, and Prince Kai is forced to hand her over and agree to marry the Lunar Queen to avoid a war.
At this point, Cinder finds out her true identity and realizes that, as Princess Selene, she is the only one who can truly stop Queen Levana. Rather than remain passive, Cinder embraces her powers out of necessity. She realizes that she'll use them—though she finds them immoral—as long as they keep her safe on her journey to meet with Dr. Erland. Though she doesn't accept her birthright as the princess, she does accept her powers, and this puts her on track to eventually accepting her position as princess and working to fight the Lunar threat in later books in the series.