 If you liked
Koizora: Setsunai Koimonogatari
|
 ...then you might like
Taiyou no Uta
|
Both mangas have a tragic romance theme. Both have a person that has a disease unbeknownst to the love interest, but is soon found out. Both couples separate at one point, but get back together. However, that person's health deteriorates, and it ends tragically. Taiyou no Uta is much more short and lighthearted, whereas Koizora: Setsunai Koimonogatari is much more long and much more mature.