I just found the anime for this without knowing it was BL or even reading the summary. It's surprisingly similar I'm terms of the panels of the manga...
I usually never read or watch sad stories because it hurts so much. Anyways, I came here to read up to the same point in the anime (ch 11/12) cuz I don't wanna know what's coming.
I haven't even gotten that far in the story but I feel the same as you. They barely showed Yuuki and Mafuyu, but the scenes and the timing makes such an incredible impact. And the song... Bro I've watched the singing scene three times today and I just keep crying.
I'm still crying. My heart hurts so much. How could Yuuki do that!? Why did Mafuyu have to say that!? It's not your fault! But it hurts me so much to see this situation. I need therapy.
Oh no...I already own a Pomeranian... Guess it's time to join a band

I think, as a reader, I'm the one who can't move on from Yuuki. I mean, I love his character so, so much. I know that he's dead and I know that what we see in the mangas are only memories of him but even so, I just can't accept his death. And to be honest, even if I want him to be happy, a part of me is really sad to see that Mafuyu is finally starting to move on. But once again, I'm just sad because of my own inability to accept Yuuki's death. I want Mafuyu to be happy but at the same time I don't want him to be happy without Yuuki.
Argghhhh!!! It's so complicated... I feel like, I'm too invested in this manga and that makes me mix my own feelings with Mafuyu's one's. To the point I'm starting to wish for him to stay unhappy, even though I don't want that to happen either. I don't know. This mangas makes me feel so much... This is such a masterpiece but really... It truly broke a little part of my heart.