Yukishimako

Its about two deities(?) and the ml is undergoing through a challenge where he needs to survive 3 lives in total if I remember correctly, and the fl follows him and helps (?) as she likes him. Their first life was in a Korean(?) historical setting, where the fl became his mentor(?) and ml a student(?). Their second life was in a modern setting and they reversed genders accidentally while retaining their memories as a diety and past life and both died on a subway train. Now the third one I don't remember. What I remember of their characteristics is the fl had white hair as a diety I think... While the ml had long black(?) hair?? Help me find this guys, I wanted to reread it but my lists are gone ╥﹏╥.