 If you liked
Yozora no Sumikko de,
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 ...then you might like
Sayonara Game
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Both manga have a school boy who has a crush on his senpai, that he only gets to confess years later, when they're both adults.
[MINOR SPOILERS]
Both manga also feature someone related to the senpai, who not only develops a crush on the said kouhai, but also ends up confessing their feelings to them. Only to be rejected, ofc.