How we cite our quotes:
Although it was she alone who saw this magic, she did not wonder at it. She knew it was all due to Sula's return to the Bottom. It was like getting the use of an eye back, having a cataract removed (1937.55).
Sula is so important to Nel that she is practically a part of her body. When Sula leaves, it's as if she takes a piece of Nel away with her, and her return to the Bottom signifies Nel's return to completeness.
Here she was in the midst of it, hating it, scared of it, and again she thought of Sula as though they were still friends and talked things over (1937.189).
Although Nel might not realize it, we can see that what upsets her most about the affair between Sula and Jude is that she no longer has Sula to depend on.
"I was good to you, Sula, why don't that matter?" (1940.70).
Nel simply cannot comprehend how Sula could betray her after she has been devoted to her for so long. She believes that being "good" to Sula should have ensured that Sula would keep her hands off Jude. But Sula constantly questions what being good means and who is actually good. It seems that Sula feels that being good means recognizing and appreciating the inner "glory" we see in the epigraph, not just doing what everyone expects of her. And since she doesn't see her affair as a betrayal, the question of being good or not doesn't appear to be particularly relevant to her in this situation.