I don't know if there's a way to fix it short of sorting out your regrets and not just possessing some kid anymore, but step one is actually feeling guilty. Sure, one could argue that you don't technically accomplish anything purely through the guilt but acting like you're fine is pretty callous and selfish. Especially since he pretty much says that he understands he can stick around as long as he has regrets and that the last time he felt as if he had fulfilled his purpose (ie his novel became successful) he faded away and the kid came back (years after the fact in his case). That means he KNOWS he's in control of this. So his "if this is what fate wants who am I to argue?" attitude is totally obviously self-serving.
Takae seemed to respond subtly negatively to him accepting that he thinks he's supposed to be here/allowed to be here as long as he still regrets something and seemed genuinely freaked out by the fact that she's stealing Marika's life from her. So I think that while we may not see the author guy confront his own problems, I believe that the buildup on her end will be using a bit more time to sort out her life and then the ending will be her letting go and saying goodbye so that Marika can have her own life back with a better relationship with her mom and with new family friends willing to look out for and help her in Takae's family.
Help the husband move on and accept that he can't pine after her regardless of whether she's gone or in the body of an elementary school girl (cause that's super freaking creepy, sitting there and staring at the proverbial clock until the moment it's not longer legally/morally objectionable to date/marry her) and push him to give a chance to a second love with his coworker who is, by every account and observation, really nice. Help Mai get ready for her wedding and moving out to start her own life without feeling like she has to stay and keep her dad afloat while he drowns in his misery. Help Chika figure out proper balance in her life so that Marika has a stable home environment and the two of them can patch things up.
That's the path forward and yeah it's a bittersweet ending for Takae to let go and move on but that's kind of the whole point of us finding out that this is not true reincarnation but rather is more akin to possession. Because most reincarnation stories tend to go one of three ways 1) The person who is reincarnated is always aware/in control and it can be argued that there never was another version of them to begin with, 2) the person who is reincarnated is still basically the same person before and after getting their memories back so it can be argued that there's no replacement, just possibly greater understanding of how they have the same thoughts/feelings/likes/dislikes as their past lives, or 3) the person who gets replaced when the reincarnator gets their memories was a massive asshole and therefore we don't feel bad about them being swept away because they didn't deserve their life (ie how it goes in most "I was reincarnated as a villain/villainess" isekai type stories. The moment they told us that Marika was a good kid who didn't deserve the crap that happened to her and it was clear that Takae was not simply Marika with more memories but rather a different person at the controls of the body it became clear that there was no endgame with her (Takae) still inhabiting the body without it being really dark.
