It probably boils down to being a hopeful kid, versus an adult scarred by reality.
She was ridiculed/ostracized her whole life, which only got worse after Elis was exiled - the presiding Queen also hated her guts, presumably because she blamed Cass in large part for it. It’s not an easy life being hated by the Queen of a kingdom, and only tolerated by your father, the King.
Due to this, she probably feels some sort of insecurity towards Elis: wondering if he truly loves her as a woman, or just someone who warms his bed. It’s clear she loves Elis romantically, and that she wants more from their relationship, but can’t bring herself to say it out loud or admit to herself, because she feels it would undermine her position and/or reputation.
I expect her demeanor to relax once Elis officially makes her his Queen, and she gets definite confirmation about Elis’ feelings.