The great and powerful Trixie looked herself in the mirror in her wagon, and for the second time that day, saw herself for what she really was. A failure.
She'd wanted with all of her heart to be a scientist. She'd wanted to work in laboratories in Canterlot, and make life better for ponies. She'd wanted to use magic for the good of ponies everywhere. But she failed.
The Canterlot Academy of Sciences wasn't particularly rigorous, but Trixie was simply not suited for her passion. The mathematics of it eluded her, and the theory she found tedious and boring. She watched her dream slip away before her as her failing grades closed in upon her. She drowned in sorrow.
She had taken up performing small acts of prestidigitation for the amusement of foals. That is when she knew her life had hit its rightful niche: rock bottom. She inflated her ego through pomp and showmareship, and she began to forget why she felt worthless. But when she looked in the mirror, and she saw her robe and hat in place of a lab coat and goggles, she could see through her own façade.
Deep down, she knew what she really was.
Worthless.