
Well I mean, both people were being severely hurt in that relationship. One, being hurt physically by the one they love and two, being hurt mentally by knowing they're a deranged being who hurt the person the loved. Honestly I prefer the relationships they have now since it's a lot more healthy for both parties.(づ ̄ ³ ̄)づ

They didn't end their relationship please! It said the manga was still ongoing. Shuuna's transfer was a fact so he had to move out to Osaka, they're mantaining a long distance relationship for now, otherwise, why they would have phone sex?
http://ba-raw.livejournal.com/tag/hadakeru%20kaibutsu

It doesn't make me unhappy in the slightest. That relationship was horrible and at a point of no return. I love them with their new partners now...they both are MUCH happier.

To be honest, I also found Hayashida and Yumi's relationship quite interesting, and I believe it could have worked out under different circumstances. One in which Hayashida had achieved some emotional stability and confidence in himself, and one in which Yumi had given up on his old crush and had opened his heart to the possibility of new love (of loving back Hayashida).
But given the current timeline, I would hope they would never get back each other. Hayashida, looking at Yumi, would see his old ghosts rather than Yumi himself. And believing that he could never get past his horrible, abusive self, he could fall into a spiral of self-deprecation that would tire him out and ironically push him back into that "comforting" (for a lack of better words) cycle of domestic abuse. And Yumi would still cling onto Mayama, rather than coming around to love Hayashida. If anything, he might undergo character regression and accept the abusive relationship for what it is, because it's more convenient to believe Hayashida's former kindness will return than to try and "fix" his character. The manga has shown both characters developing and moving up from their old scars, but the scars will remain. And if you continue to pick and peel at the scars, you'll reopen not only old wounds, but also new ones. I don't think either character has that much emotional strength to reestablish a healthy relationship.
And even if they were healed and able to treat each other cordially, would they really be able to open up to each other? Even if they didn't become an abuser/abusee again, could they really learn to unconditionally trust one another, and completely bury their morbid past? I personally don't think so.
i miss hayashida-san and yumi .it makes me sad knowing they're not together anymore ;____;