Mugen no Kyoukai ~Kiseki no Umi~

Hero is an ordinary student. After getting in a traffic incident he starts to see events of his previous life in dreams. His previous life was warrior guardian of dream world where all the myths originate protecting it from nightmare monsters.
One day he gets to know that the seal of the dream world is broken, and nightmare monsters threaten to destroy real world next. He needs to regain his love reincarnated in this world and fight invasion with the power of love.

[From vndbreview]