When she's just a Soul, without a Host, she isn't really aware of anything. She's counting on that if Doc keeps his word and leaves her to die, because it means she won't feel anything as she dies, and what with the strong emotions in Melanie that seems fitting somehow.
But right now she's in Melanie, with all these human emotions like fear and love and anger. She can feel Melanie's anger and sadness at what Wanda has chosen to do. She can feel her own fear, if it is her own, at the thought of dying. She feels her love for Ian, and Melanie's for Jared, and then little Jamie.
She thinks the idea of not being to feel anything is what frightens her the most.