I can tell this is an unpopular opinion, but while Chie x Ai is my favorite couple in this series, Shizuka is my favorite character. She's complicated, and at times not a good person, but that's what makes her so interesting. In my opinion, a character isn't truly human until they do something that makes you hate them.
Despite how hard she's trying to be a good person and a good partner for Mio, she's still struggling to get over someone she loved so dearly in the past. This isn't easy for everyone, and it's clearly taking a huge emotional toll on her and, possibly soon, Mio as well. As much as I want to say she doesn't deserve Mio and that they should break up, I also want to see her talk to Mio about her problems so they can figure out how to work through them together. Given how much they love each other, I feel like they'd both be up to the challenge.