It’s from the film “Janghwa, Hongryeon” or " A Tale of Two Sisters"… It might be called “돌이킬수 없는 걸음 (No path back)” or Epilogue. The soundtrack seems to be out of print.
Can you read Korean? You can look it up on naver.com (one of Korea’s big search engines) and download it from there. If you need help with that, let me know.
You can view the track list here: VIBE (바이브)
The one you mention is #13.
If you install the Naver music player, you can listen to the tracks.
By the way, I love this movie. The visuals are stunning, and the music is beautiful.