I think Amethyst meeting the rest Amethysts from the Prime kirdengarten and being accepted by as one of them was what finally made her self-hatred and insecurities finally go away.
She got most her character development in Season 3, but i going to the human zoo was the "final step" for her arc.
She was clearly very happy to meet them, and knowing they considered her one of them despite her short stature was very important for Amethyst.
Keep in mind that after this episode, Amethyst gaves up wrestling since she doesn't need it anymore to feel good about herself, so being accepted by her fellow Amethysts clearly changed her.
Gifs and images source: Steven Universe Wikia









