My personal fan theory for Opaline Arcana's true backstory is that she was friends with Celestia and Luna but that changed after the two were chosen to be royalty. MLP has a theme of ponies becoming bitter after a friend leaves, misunderstanding it as "they think they're better than me now". Just look at Starlight and Moondancer.

In Opaline's little story, she portrays Luna and Celestia as uncharacteristically snobby and superior. That's probably how she viewed them after they left her all alone in Skyros. After all, why be friends with a nobody when you can go off to be royalty? I tried to make it look like Opaline is trying to do magic but only weak flames are coming out, making her cry. She feels like she'll never be enough. She wants to prove that she was just as, maybe even more, worthy of becoming royalty as her former friends.

Under all of her cruelty and lust for power, She's still a child that feels abandoned by those who she thought would be with her forever.