|I'm looking for romance manga where a lover's personality changes for better or worse. The cause can be anything from black magic to amnesia to just random will to change.
I can think of Diamond Dust (Kang Hyung-Gyu)
Spoiler (mouse over to view)
Brain tumour changes his playful personality into a bitter one but she sticks with him until the end.
Noting manga like that Six Half or Lover Discovery don't count because they are not actually in a serious relationship when they experience personality change.
Particularly looking for manga where the lover goes insane.
Horror manga are so unpleasant. Rarely scary. Just unpleasant and gory.