She looks like she's about to burst into tears over admitting it, and Cassian's heart hurts for how upset she is. He reaches out and cups her cheek, because he's trying to process this information while simultaneously wanting to wipe that expression off her face.

Jyn is a princess. His girlfriend, his best friend, is the crown princess of Denmark, meaning one day she is going to be a literal queen.

He feels a little dizzy, honestly, but he doesn't let go of her hand or her face, even if his mouth opens and closes in the shock of it. He bursts out a nervous laugh. "How long have you known?"

There, he can make a joke in his attempts to diffuse this tension, because he doesn't want her to flee and he needs to give himself a few more moments for this reality to continue to sink in.