I read it from my library years ago, and it was one of those fairytale retelling type novels. I thought it was by Gail Carson Levine along the lines of Fairest, but I can't find any of her books with a matching description. This particular one was a combination of cinderella and sleeping beauty. I can remember like the whole story but none of the names or stuff I can use to easily google it.
The main character starts out as an overweight and lazy/antisocial princess with an evil-ish stepmother. She doesn't want to do stuff and so gets locked away in a room which she discovers has an entrance to a secret passage that leads to a tower with a magic book. The book teaches her enough magic so that she can clean the place up but won't let her learn more until she does. She also gains access to secret passages that are marked by crest symbols in the hallways behind doors and explain the extra wide walls in the castle, so she raids the kitchen that way. Her step mom wants her to marry a prince who she thinks is gross.
She decides to run away and splits/makes a copy of herself with magic, leaving one half comatose in the castle while she's out and about. She ends up going over a mountain and gets stuck working as a servant girl in the princes camp as he prepares to make war on her kingdom. She eventually end up back at the castle and rejoins her two halves just in time to stop the prince from giving her the kiss of life and insults him.
She also takes one of her stepmom's dresses with a dragon horn looking headdress, shoves a broom down the front, and goes flying over the princes soldiers to scare them thinking she's a dragon that had been terrorizing both kingdoms. They each thought it was the other one and there was no dragon which is why they were going to fight. She tries to magic up rocks to dump on the soldiers but ends up with mud instead, making them think the 'dragon' pooped on them. The prince and her eventually end up finding the real dragon and she tricks it into eating ice by making the illusion of it being her and killing it. They realize they actually like each other and she ends up taking her kingdom back from her step mom.
I'm pretty sure the princess was named after a virtue, like Prudence or something.