They went different paths based on what they wanted. Cope wanted to fly for the sake of feeling free. Tewy wanted to lead and fight in war. Which is why Cope says that changed her because before she would never harm someone.

They diverged likely prior to desertification as her being in a family that founded their Air Force and the war with Yuktobania. Which was the cause for them to industrialize in the first place. And the same war which pushed her to fly to fight.

The original question was a joke lel but I can expand a little bit on what u said

I think Tewy didn't really "want" to fight/lead Sotoa into war but was moreso compelled to do so because of her noble upbringing. She had to do it because she was practically born into the role

I also think that's how and why Tewy forgot about cope. Either by active conscience or by the natural passing of time, her feelings for Cope dulled further and further, and her only link to a childhood dream died along with her memory of him.