I don't blame her for being upset.
Aqua falling into the fountain and recreating the CG scene was a pretty big indicator for her that the events of the sequel can still occur, regardless of how she tries to change it.
Even when she knew how the game would end in the first arc and basically changed her entire character, she still got screwed over with the same rejection scene. She probably feels like there's a force stronger than she can fight. But at least back in her home country, she had family and friends to support her. Now she's in a brand new country and only knows like 4 people, one of which is her new rival (and to make things worse, her rival is a genuinely lovely girl with A LOT of power backing her up).
Add in the fact that she's still a teenage girl, and you obviously see why she'd be having panic attacks. Even if Aqua tells her that he'd never leave her, she's been rejected and humiliated by her fiance (who was her childhood friend, mind you) before, purely because of the set up of the narrative. In her mind, she's probably thinks the same will happen again, no matter how hard she tries to fight it, except this time she's genuinely in love with Aqua and she doesn't know the storybeats so she's entering the situation blind.