She's basically had to put up with him treating her as a slave for 10 years, with everyone else being too afraid of him to do anything about it. Which included her running around every day trying to clean up his messes, because he's a rude, stupid, ignorant, mean spirited moron, as well as having to do his chores for him.
When he was thrown out of the special class -- because again, he's a lazy mean spirited moron -- he tried to assault her with a knife and only survived because the First Prince stopped her from defending herself. His older brother, the first prince, is just as bad but in different ways, being a manipulative high spec prettyboy type.
All of which means that the second prince calling off the engagement finally let her escape from that hell, although the Second Prince might not have been allowed to call off the engagement since it was set up by the king, and their kingdom practices royal polygamy, so it might be a situation where she's stuck still engaged to him as his second wife or something.
Bonus: The Baroness slut who faked all the harassment whispered "baka" at the MC when it was all being announced, so she's on the long term shitlist.
The MC was always a berserker type but settled down because of her father pushing her to do so. She kept up on her training, both in martial arts and magic combat, and the first prince teasing her caused her to go hell for leather training in both, which came in real handy, as you can see.