This might be a long shot but I would so much like to know what this story was called so I can read it again. I think it's from the 90's and I read it in an anthology--it might have been a year's best in fantasy and horror.

There's a man who has magical abilities but no conscience/his mindset is rather simple. So in his world they pair him up at a very young age with a woman who trains him to love and obey her, and she lives with him his whole life and acts as his conscience. He can only do magic if she says it's ok.

I remember a part where a villager comes to him for help and she's not around. He grants the wish but takes it too far--and he finds enjoyment in being perverse and not just using his magic for good.

The ending is that she finds out she is dying and she knows she can't leave him by himself. She has sex with him one last time and then she kills him. I remember it being really poignant and sad.

Either he or the villagers might have called her Mistress? Or was it something weird like Goodwife or Gottwife?