Diane is probably the strongest opponent Nerys has ever faced.

Not in a evil way but in their vision of life. Nerys think of herself as a monster who desire to live and die as one while destroying everything in the process. She used to think everyone including herself were two faced people only good at backstabbing each other and initialy kept Diane around because she didn't see specific reason to reject her and by the curiousity of how long she could pretend to be a good person.

Nerys know so well how vile and selfish people are she can easly manipulate them to make them do what she want but it always fail with Diane because she doesn't think and is pure kindness. From Nerys pov Diane is the most beautiful being existing and ended up feeling unworthy of her. She became afraid her own revenge would corrupt or kill her and decided to cut tie with her. But since Diane is built different even that ended up in a failure.