I can't see why that's a problem. Eri is obviously the type who values her best friend more than her partner. If Fuyuki was not in love with her, there wouldn't be any struggle in this. They can prioritize each other first while having their own mate.
It's a problem because most people aren't going to be OK with coming second best in the eyes of their life partner, especially if the relationship is as intimate as the two of them have. Even if they both somehow found unicorns, it would still be an issue because Fuyuki is the sole source of all Eri's happiness. And that's just not tenable. If you've ever seen or been in codependent relationships like that (romantic or not), you know they never ever work and usually implode spectacularly because they're deeply emotionally damaging and — in many cases — also abusive.
While I don't think Eri will end up going quite so far, the fundamental struggle she has is that it's deeply difficult for her to connect with other people, and she's fearful of doing it. In her romantic life, this takes the form of letting men use her/say/do whatever to her in exchange for their continued attention, with no hope for emotional fulfilment. So Fuyu and Fuyu's promise of unconditional love remains her constant wellspring of affection, attention, and validation as a human being. Like........her self-esteem is shit. Which is why she's devolving into dramatic meltdowns when she perceives her place in Fuyu's heart is even slightly threatened. And Fuyu is aware of this, which informs a lot of her hesitancy. There's zero sense in unloading her feelings on Eri when Eri can't even return them, and the likely scenario is that it forms a rift between them that causes Eri to fall apart. No matter how you look at it, it's not fair to make someone your emotional caretaker. Yet even so, Fuyu considers her feelings the problem. Hence her solution being "get over it" instead of "confess."
Never heard of that other story, but tbh I don't think all narratives with similar themes need to follow the same plot beats to be good. The characters in this story are all acting believably for their personalities and circumstances, and they aren't going wildly out-of-character to keep the miscommunication going. So for me at least, that keeps it from being frustrating. What can I say, I love mess. 🤷🏿♀️