I just can't get over that complete and utter lack of self-accountability. It disgusts me, having dealt with people like that in toxic relationships before, and I don't feel like I'll be able to see Kaoru differently unless the author addresses this issue at some point.
I think it's worth remembering Mayoi's initial reaction to Kaoru and how humiliating it was. That was a fistful of karma, particularly for someone like Kaoru, and having to deal with Mayoi is forcing Kaoru to grow - albeit, slowly and unevenly.
In general, I understand how you feel about Kaoru and Shizuka. For what it's worth, this is a more accurate reflection of real life. In stories, people always get their second chance to resolve things. In life, usually you screw things up with one person, and at best you learn your lesson with someone else and do your best going forward. As for the other person, new love has a way of cleansing old hurt.
Hopefully Kaoru learns her lessons with Maoyi. And if that happens, well, since this is a comic, maybe she does have her moment of resolution and closure with Shizuka. If she does it will probably be more for Kaoru's sake than for Shizuka's.