The major frightening truth that Kira discovers is the truth about how brutal and manipulative the leaders of her society are. She learns that they (even when they seem to be nice and caring like Jamison) are willing to go to any lengths to preserve their control over society.
For example, Kira learns that there are really no terrible beasts out in the forest waiting to kill anyone who ventures out there. Furthermore, she learns that people who tell the truth about that tend to end up dead. This helps her to understand that the Guardians are really just using whatever means they can to control the populace. They are willing to lie and to kill to do this. This is surely a frightening truth.