I agree with
@anf .
Mio is half romantical/idealist and half very mature, even without experience. In the end, she understood that Shizuka have what I personnaly name a "ghost" (more precisly I call it a "fantôme" since I'm french). A ghost is a souvenir of a person we have a relationship with, wich is kind of unfinished in our head. So the souvenir is haunting us because it's not concluded. If the person still extist, we can be confuse about the past and the person. I'm pretty sure I'm not very clear
sorry. In others words, Shizuka didn't realy conclude her relation with Kaoru, not in the sens she still do love her because she do not, but, for exemple, she never had the occasion to tell her something she wanted or something like that. Or it ended not the way she would have want. So she is confused and that made her "messing around". In my life, I had few times been haunted by those kind of ghost. And I telling you guy, it's not about still being in love, it's about the possibility to turn the page, and that can make the next relationship a little odd in a first time. Mio understood it, accepted it because Shizuka had to go that way, maybe she was not forced to kiss Kaoru but in the end this only accelerate the recovery time. Shizuka was damaged by her toxic relation with Kaoru, no one can, I mean I, I can't blame her for that. The fear and the conscience of what she did to Mio will almost entirely finish the process of recovery.
When
@anf says Mio was a substitute, I agree and I will go further : she was a necessary medication that prepared Shizuka for a real new and honest relation with Mio. It's because she understood it in her heart and by love to Shizuka that Mio, in the end, forgive her.
Well of course it's just my interpretation.
I think Torajiro is a very goog psychologist.