Promising one thing doesn't mean you're gonna keep it though.
I feel like without any closure to her and Lee Hwa Hayoung won't be able to move on and actually trust her new partner. And her and Lee Hwa are equally to blame here since they simply refuse to resolve things once and for all.
Seo Woo is honestly the only one with real character growth as of now
This isn't character regression. This has consistently been Hayoung's problem since they started dating. She is -terrified- of getting into a fight. She still doesn't know why Lee Hwa left her back in the day, she never learned to move past that trauma, so she shuts down and runs away any time a serious subject comes up. She responds to Seo Woo's probing about living together with silence. She avoids talking about her past with Lee Hwa. She shuts down every time a serious topic comes up.
Of course she didn't tell Seo Woo about Lee Hwa's mother. Because talking about your ex could start a fight and she CAN'T start a fight because what if they break up over it?
Even the promise to tell her things was just a lie intended to mollify the current situation. She's been in full retreat every time anything serious comes up. The problem is, by trying so hard to keep Seo Woo placated, she is endangering the foundations of their trust in each other.
It isn't character regression, you're right. I am not very good at writing serious comments. But nonetheless the drama of the scene was definitely forced.
Ha ha, I sure love character regression! Nice job going from promising to tell Seo Woo things to not telling her about Lee Hwa being there when there was literally no reason not to, Ha Young. I blame the writer on this one.