Our faults irritate us the most when we see it in others.
This is what I read somewhere a long time back. It was so true for Ayesha going by yesterday's episode. She kept imagining the entire Shah family laughing at her plight. She envisioned Prachi clones appearing every where and ridiculing her, telling her that she will never get Milsi and that she will have to live in the same house and see them happy together, that she has lost and Prachi has won etce etc. These are the things she herself would have done had she managed to get Prachi out of Milsi's life and get Milsi to marry her. That is why she imagined Prachi to behave in the same manner. And it sure the hell not only got her irritated but was driving her insane.
To add fuel to the fire, when she saw Prachi decked up as a bride, smiling at the mirror, she thought that she was smirking at her. Prachi, I think, was not even aware she was standing behind even though her image was very much on the mirror. She was totally lost in thoughts of the impending wedding to take note of her surroundings.