"I'm a special, irreplaceable existence."
...Yep. That's pretty much the problem with her head exactly, it seems.
It seems like Cecil's emotional stunted nature is the key here. And there's something about some kind of fated maiden? Since Cecil doesn't know about it, I doubt it's an in-universe legend or saintess thing, unless it's super old.
Uh....not much else to say. This is mostly an attack on the crown prince, and grounds for execution. She's selfish and entitled to the end.