She based her whole life around her vision of who he was and now it's being shattered around her.
She based her entire life around what she assumed someone was like. Now she's learning that he's an actual person and not this crazy ideal she's built up. It's her own fault for building up someone she doesn't know at all into this paragon of what she wants to be.
Try to put yourself in her situation she's a little girl raised and living in a brothel where she sees all ugliness coming from people, since that's not a pretty place for sure to be for a little girl.
Now she is visited and trained by some guy who is acting decent enough for her to admire him as a father figure, then we can also assume that her mother too speak higly about the guy enforcing her daughters visions, wanting to give her daughter some kind of moral compas figure in life.
So Bern build her life upon this idealised version of perfect father knight wanting to follow in his footsteps, she already got a cold shower from the Knights where she had to rearrange her ideals in life, now to fully shatter the picture she meets her father who was not a perfect knight at all, who now is just a fallen knight, who drinks, and has everything in the ass, and now will tell her the real story.
Life changing much?
I can't blame Bern here, she is desperately holding to last good moments from her childhood, even if they were not real, they were real for her.