Didn't Shoujo moved in with Mito back in Secret XXX? So why is Mito discussing moving together just now?
I see it kind of like a flashback in a sence or at least another perspective. In Secret XXX, it just skips to when Mito and shoujo live together. In this story, it shows that while mito and shoujo were in the middle of moving, this discussion came up. If that makes more sence at all. (Sorry im not that good at explaining)
Hate saying the b-word but wtf is wrong with the ex-gf?
If I had Kashiwagi who's good-looking with a good job, good sense of humor, let me live in his nice place and buys me what I want, I'll keep him forever!
she probably had commitment issues
I think she felt like she wasn't good enough for him because he was always selflessly doing so much for her, and so she self-sabotaged out of guilt. Self-fulfilling prophecy and all that as she turned into someone who definitely did not deserve him.