Mikazuki to Nagareboshi

From Whitelies Fansub:

Hasegawa Mitsuki, a college freshman. Mitsuki has been thinking about the same person since she was 16 years old. That person, being 18 years older. A Coffe Shop Manager, Asai Ryusei, a divorced man with an only son.
To Mitsuki, Ryusei is a really important person but, her romantic interest for him is slowly…?!