Don't touch what is mine
In Kaguya's mind world, after falling under Priscilla's charms.
She was trapped inside the illusion; she was experiencing the same thing repeatedly. She was on her knees, apologizing over and over again.
She might appear stronger than most teenagers, but she was still a living being who could make mistakes and feel guilty. No matter how strong one mental strength is, if you believe that you are guilty of something, you're persuaded of that and refuse to forgive yourself for this mistake, you'll be trapped in your illusion forever like an infinite loop; it would become your hell.
It was exactly what was happening to her; she was persuaded that it was her fault if Mikasa was dead; she felt guilty; she was asking Mikasa to forgive her, not knowing that she should forgive herself first. She couldn't forgive herself as she was the one who suggested going out that day.