I don't think Mafuyu is using Ueno to get over Yuuki. Yuuki IS irreplaceable to Mafuyu but that doesn't mean he cannot fall in love with someone else. The time he shared and the memories he made with Yuuki are forever going to be with him and they're irreplaceable. Yes his heart is broken and he misses Yuuki but other people in his life can heal him — his friends, his family. Does this mean he's "using" them? We as humans move on from the death of our loved ones. And that's just how life is. We continue to live making new memories with other people in our lives. But this doesn't make the memories we made with our departed loved ones are any less significant.
So Mafuyu CAN love a new person. And I think he also knows very well that Yuuki is irreplaceable to him, that he still likes Uenoyama but they're both important. He says so to Hiiragi at the end of Ch 11. http://www.mangago.me/read-manga/given/mf/c3b7169df64ad239/Vol2_Ch11/pg-34/His song was a closure, a signal for him to move on. But he’s never going to completely “move on” from Yuuki. The feelings of grief, loneliness(and even happiness from their good times) are proof of how he was an important part of his life. But, that doesn’t mean, he should never be allowed to love a new person.
Mafuyu is actually very "mature" in this regard—he knows where his feelings stand. But he's also human. And he will struggle with his emotions (like his fear of losing a loved one because of a lack of communication), his relationship with Uenoyama will have ups and downs. But he's still moving forward. He's making music, he's making friends, sorting out his relationship with his old friends (hiiragi and shizusumi).
This is why I love sensei's work so much. The characters are so human. They have their struggles but they're moving forward and it pushes me to do the same. My goals seem reachable even if I'm struggling.
I don't think Mafuyu is using Ueno to get over Yuuki. Yuuki IS irreplaceable to Mafuyu but that doesn't mean he cannot fall in love with someone else. The time he shared and the memories he made with Yuuki are forever going to be with him and they're irreplaceable. Yes his heart is broken and he misses Yuuki but other people in his life can heal him — his friends, his family. Does this mean he's "using" them? We as humans move on from the death of our loved ones. And that's just how life is. We continue to live making new memories with other people in our lives. But this doesn't make the memories we made with our departed loved ones are any less significant.
So Mafuyu CAN love a new person. And I think he also knows very well that Yuuki is irreplaceable to him, that he still likes Uenoyama but they're both important. He says so to Hiiragi at the end of Ch 11. http://www.mangago.me/read-manga/given/mf/c3b7169df64ad239/Vol2_Ch11/pg-34/His song was a closure, a signal for him to move on. But he’s never going to completely “move on” from Yuuki. The feelings of grief, loneliness(and even happiness from their good times) are proof of how he was an important part of his life. But, that doesn’t mean, he should never be allowed to love a new person.
Mafuyu is actually very "mature" in this regard—he knows where his feelings stand. But he's also human. And he will struggle with his emotions (like his fear of losing a loved one because of a lack of communication), his relationship with Uenoyama will have ups and downs. But he's still moving forward. He's making music, he's making friends, sorting out his relationship with his old friends (hiiragi and shizusumi).
This is why I love sensei's work so much. The characters are so human. They have their struggles but they're moving forward and it pushes me to do the same. My goals seem reachable even if I'm struggling.