This is honestly really depressing. I remember seeing this at the manga section of B&N a couple years ago but it was wrapped in plastic so I couldn't see what it was (back then I was innocent and did not know why it was wrapped in plastic LOL) .
if you've seen the anime I recommend reading the last two chapters, its a bit different from the anime and IMO way better than what they decided to put. Absolutely heart wrenching though, this left me empty for days...
The Story of a Girl with Sanpaku Eyes