Hestia had a good public image because of her family's good reputation and she has connections everywhere because of her family's large connections so they could easily cover up all her mistakes.
Unable to win you over, Hestia saw your relationship with your fiancée. Hestia was envious. Why couldn't you be like that to her? Why can't you see her in that way? In the end, Hestia resorted to her last resort, which was keeping you in a mansion just to herself. She knew it was wrong but she was desperate at this point.
At first, you rebelled and rejected her in all sorts of ways. It hurt but Hestia wasn't going to give up so easily. She made up stories about how you were hallucinating and she was your fiancée, not that other woman. With the help of medications, you were forgetting.
"Don't forget to take your meds." She says sweetly, a smile on her face.