Collins describes Rachel Verinder's complicated love for Franklin Blake (which she is implicitly confessing to Godfrey Ablewhite in the passage quoted below) through a paradoxical metaphor. In Chapter 5 of the First Narrative, Miss Clack overhears Rachel ask Godfrey what he would do had he ever loved someone who was "unworthy" of him:

And, suppose, in spite of all that – you couldn’t tear her from your heart? Suppose the feeling she had roused in you (in the time when you believed in her) was not a feeling to be hidden? [...] How can I make a man understand that a feeling which horrifies me at myself, can be a feeling that fascinates me at the same time? It’s the breath of my life, Godfrey, and it’s the poison that kills me – both in one! 

Rachel speaks in general terms here, but the reader knows she can only be referring to the painful situation she's facing with Franklin Blake. As she believes he stole the Moonstone, she thinks he is "unworthy" of her, but can't help loving him anyway. When Rachel says the feeling that "horrifies" and "fascinates" her is "the breath of my life" and the "poison that kills me—both in one!" she is using a paradoxical metaphor to explain her confused feelings. The metaphor refers to her simultaneous joy in her love of Franklin and her disgust at what she believes he has done. She is horrified by her love (it "poisons" her) and also sustained by it (it is her "breath," the thing that keeps her going.) This metaphor is paradoxical because these emotions would usually cancel each other out, and yet she feels them simultaneously.