SHE'S TRYING but he keeps on stopping her from talking further. He only listens when it's something that's related to the problem like being the child of god.
But that's because she always starts off by saying "Please believe me, I am Princess Luciencia" and stuff like that. If she just started abruptly by saying something like "March 25th, one year ago, we were taking a walk in the garden when I told you so and so" at least he would hear something of substance other than her just insisting she's the princess without proof
That's true but she would be treated as a spy again. He would only think he investigated him or maybe someone stalked him that day. It doesn't seem like they have a secret between each other, too.
Bruhh why can't she just narrate past things that happened when they were alone with each other to prove herself?? She could tell him something only they both know, or childhood stories, or casual conversations they had. That would at least erase some of the doubt from his mind.