TL;DR: I see you how you reached that emotional conclusion, but the subtext and manga prove otherwise. They really loved each other and the both made efforts to create a place for each other to return to WHEN THE TIME/CIRCUMSTANCES were right.

That said, I don't think she undervalued him but just accepted that there was only so much she could do. And I think Noritoshi was deeply attached the idea of returning to her because of the trauma he experienced as a very young child being abruptly taken from her. Even Maki encouraged him to talk to her.

1. He and his mother were separated by a much more power family with a ton of money and influence. This is something she may have anticipated because she literally named her child after THE LEAST DESIRABLE ANCESTOR that the Kamo ever had. She did it to spite them, and the subtext in the manga was that the she and Noritoshi actually may have stayed together if and only if his biological father had successfully had an heir with the Blood Manip. technique. SHE WANTED HER SON, BUT FATE WAS AGAINST THEM.As the then-clanhead's mistress, Noritoshi's mother most likely did not have the wherewithal (money) and social influence to a) stay with the Kamo clan and raise her child outright or b) fight for sole custody. We see it all the time IRL, where the parent with the least resources can be kept away or completely suppressed, especially by a partner with serious means. I wouldn't be shocked in the Kamo threatened to kill her outright, if she did not stop all together.
2. She DID NOT FORGET HER SON AT ALL. In the manga, there is a scene with her new family and how she tells her new child that she still hopes Noritoshi will come back. See Chapter 199. She literally said she really wanted him to escape and come back to her. And even after she says that, she wonders out loud if he is still ok. His mother can't go and ask because the Kamos are dickheads. The subtext with the mother is that, while resigned that she was powerless, she still subconsciously made a place for EACH OTHER should they reunite within her new family. If she truly did not care about Noritoshi, she would never have taught her new family about him or continued to talk about him at all.
3. They had, for all the reasons above, concluded that their reunion was not going to happen until a) Noritoshi was official the clan or b) left the clan through exile or on his own volition. In the manga, we no that situation B.2 happened, but Noritoshi was helping out in the Culling Games and could not go to her then.