Kumo no Mukou, Yakusoku no Basho, Clannad: After Story Recommendations
The side story in Clannad of the girl in the lost world coincides with the storyline of The Place Promised in Our Early Days. They carry the exact same sad empty atmosphere.
Both also regard young men going through hardships due to lost loves. Ah, serious complicated love.
Depressed main male character who monologues about not being able to move on from a past love, and that dreams feel more real than reality. Main love interest is flawless quiet girl who is basically the reason why protagonist lives.
Also have very similar questions and ideas of what the "real" world is, and a identical parallel universe theme. They both suggest that the world can dream, and that the alternate reality is the world dreaming of other possibilities. We also see moments when the worlds overlap (Orbs of light, the tower and Sayuri).
The atmosphere of both is very surreal, dissociation and