Wow they managed to make me feel bad for Hwayeon.
The anxiety of suspecting your lover is drifting to another person, and then someone texts you that they have been seen together rn this night, and you are desperately waiting for her to return to talk things out and try to save your relationship but the hours go by and she doesn't return...
The thing is...they're not really lovers? I mean, in a relationship sense, they've obviously had sex. But Jeongwon clearly doesn't think of them as "together," and can't put a name to their relationship when pressed. (though, she does admit it's more than friends)
Hwayeon is clearly obsessed with and in love with Jeongwon, but prefers to use manipulation and isolation to keep them "together" rather than just confessing her feelings and risk getting hurt. She treats Jeongwon more like a possession to be defended than a partner; it's no wonder there's no trust there, because she knows that the only things keeping Jeongwon from pursuing Jisoo were a bunch of lies and threats.
I certainly can emphasize with her, because she's clearly got some issues that lead to all the aforementioned manipulation and difficulty with emotional honesty, but I still think any problems she's feeling now are largely her own fault.