Honestly I feel like this was a well written angst story. If I were to guess Kei fell in love with the idea of Sachi. However Sachi was in some sort of relationship with an older male and was using Kei as a kind of comfort because something in the relationship between her and the man was strained. Either Sachis' age making him an adulterer, or he's having an affair. Kei realizes that Sachi is only using her after they become intimate and she realizes despite falling for Sachi, she was nothing more than a placeholder for her own frustration. So Kei attempted to change herself to suit Sachis' needs only to have gone to far and realized Sachi will never love her back. Hence why at the end Sachi thanks Kei for "keeping her distracted" but now Kei has become the thing she hates.