@inKraken
I think she mentioned it earlier. If she is found out to be as strong as she is, she fears she will be made into the "Hero", and have to fulfill the obligations that come with the title. She also fears that people will be scared to be with/near her because of her strength, and while many would try to make adjustments and to try and accommodate her power, it would just make her stand out more as being different/abnormal. Not to mention how much trouble other nobles would cause for her. She can do the same magic as the Hero, so once others got wind of that, she would be chased with marriage proposals, formal greetings, requests, and shady people attempting to make deals trying to use her. This is why she practices so hard to not crush everything she touches. She doesn't want the excessive care and attention to make it awkward for her to be with/trust people.
If people knew her strength, I doubt the prince would be able to be part of her friend group since at any moment, she could potentially crush him. No one would want to train against her, because they could never beat her, and at best would only get away with being tired. If she attacked, they could die if she isn't careful.