If you liked the Koizora manga, the Anime-Planet community thinks you'd like:
Takuma and Mayu have loved each other since they eight years old, having met while Mayu's father treated Takuma at the hospital for his heart condition. One night Mayu overhears her father explaining that Takuma will probably die before he turns twenty years old, and that Takuma doesn't even know yet. Devastated and desperate, Mayu goes with Takuma to search for a four leaf clover to wish for a miracle. When they can't find one, Takuma makes the worst promise possible: to marry Mayu when they both turn twenty. Takuma eventually realizes he cannot keep his promise and to avoid hurting Mayu, he distances himself from her. However, when Takuma can no longer avoid Mayu, what will become of the two childhood friends?
Both Boku no Hatsukoi wo Kimi ni Sasagu and Koizora are extremely intense love stories. Both will almost certainly bring you to tears. If you like one i most definitely recommend you read the other.
Both manga are beautiful intense love stories that follow the main couple through original meeting, falling in love, and all that entails for many years...
The main heroines in both stories really show growing strength of character over time, and you will cry as you follow their journey. Koizora is perhaps more focused on the heroine, whereas as Boku No Hatsukoi is more evenly balanced in POV between both the male and female half of the couple.
Both of them amongst my favorite mangas!
Ever since Kurumi's father died four years ago, she's been left to take care of her ailing mother and two twin brothers by herself. However, one day Kurumi's life changes forever when she comes across a hurt man in an alley and nurses him back to health. His name is Hakuron, and he's the young leader of the Dragon Lord Association - a Hong Kong mafia organization. After stealing Kurumi away to Hong Kong, Hakuron is determined to make her his own; and after falling in love with Hakuron, regardless of his sins, Kurumi is happy to oblige. Her new life within the mafia won't be easy, as rival organizations, Hakuron's old playboy lifestyle, his fiancée and a brand new school situation threaten to stand in their way - not to mention, Hakuron's insistence on taking what's his, even Kurumi's body...
I think if you like koizora or haou airen you'd like the other. Although haou airen is a bit rougher than koizora they have some similarities in the way they make you feel. If you read one or the other and liked it I strongly suggest you read the other. I don't think you'll regret it.