Anata Janakute mo Daijoubu de Shita. ~ Mikudashite Ita Kanojo o Ushinatta, Oroka na Ore no Tenmatsu

"But she's mine, isn't she?" My girlfriend was supposed to love me, and only me, but somehow, she found happiness with another man. Taichi is your average office worker who's been dating his girlfriend for five years, ever since they were in college. He has no doubt that Mirei loves him, but when she hints at marriage, he brushes it off, instead wanting to use her as someone convenient to do his housework and take care of his needs. However, he soon becomes infatuated with Hinako, a bright and beautiful girl he met at a mixer. Then, while on a date with Hinako, he sees Mirei with a handsome man he's never seen before. Could it be that he was the one who got dumped...?