Aah... So the idea is actually deep down in her head, she really just held herself back, huh...
You mean Kyou? It's been a thing for effectively the entire story, yeah.
She's only been "holding herself back" in the current timeline of narrative because she was convinced she had to prioritize Minori in every facet of both of their lives. Which, understandable; she was a single mother for the entirety of Minori's life up to the point they showed up on Takaharu's doorstep, and even if she had hoped that Takaharu would provide a temporary place for them to stay while she figured out next steps, there really was no telling whether it would work out at all, much less long-term like this.
But even before, she was similar. She had always loved Takaharu; they split in the first place because her own life fell apart, and Takaharu--at the time--was too involved & absorbed in his own life and his nascent art career to really be there for her. She never asked him, of course...but, this is kinda Kyou's personality in a nutshell. And so she took the first "self-sacrificial" action of the story in breaking up with him to not put any of her problems on his plate.
Her coming back to him now was because he had given her that standing offer 17 years prior, but also because she still loves him, in some fashion. She just hasn't squared that they're
allowed to "pick back up" where things were left before, because she's probably just unsure of and afraid of the possibility that Takaharu is only doing this out of obligation, or because he feels indebted, or some other "have to and not want to" reason.
We know that he's still pining for her, and beating himself up in a very similar fashion, and
also wants to continue a relationship with her. They're just both not at the point where they feel they can voice these things aloud, yet.
But fortunately, this making-up with Minori was a
big hurdle removed for Kyou's situation. Knowing that Minori will in fact be
okay, and is looking forward, means that Kyou will hopefully start internalizing that she doesn't need to put her own life on pause for the sake of her daughter--and that they're both allowed to push forward in life and pursue actual happiness and fulfillment.
The next step will be Takaharu coming to the same conclusion. He's got a couple people in his life who can help--his sister and his friend Azuma--and both have spoken to him in some capacity about a life & future with Kyou (and Minori). But he's not yet absolved his guilt about the past at this time, and will need more narrative space to do so before he can start moving toward that shared future.
I do think Kyou will get there before him, and her being earnest and heartfelt and
honest should provide a lot to push Takaharu toward forgiving himself enough to unpause his life and pursue her once more.
Though I believe that ultimately it will be Minori herself who fully 'bridges the gap', and succeeds in pushing her Mom and (soon-to-be) Dad together in full. She's the connecting character between them, as the one who represents Kyou's reservations, and a "missing piece" in Takaharu's life that could help bring him out of himself and toward an active parental role.
I don't see it happening just yet, but the groundwork's been laid, and steps are being taken, at least.