Luna will never be what Jessica is to him.

This phrase will always be like a slap in her face. Quick, but the pain lasts for a long time, and Luna just smiles, as if the pain killing her is just nothing. He treats Jessica like a princess; because she is, and she will forever be his princess. 

They whisper to each other backstage, hold hands in secret(but Luna knows this is not a secret) and their affection for each other is too much for her to handle.

It brings her down every time she sees it.