This beautiful! doujinshi is Stumbleine by hakuku-san! and there are two or three fanfics that were inspired by this story...

One is from Hoshiko2/(Kokoro2)-san!

http://hoshiko2.livejournal.com/124476.html which has a second part here: http://kokoro2.livejournal.com/8581.html # cutid1

the other is from Aria_dc_al_fine -san!
http://aria-dc-al-fine.livejournal.com/9045.html

is a beautiful and heartbreaking story! A wonderful doujinshi deserve these wonderful fanfics!!!
it's so funny that just a few weeks ago I read these fanfics!! XDDDD!!!