You can try putting an unbreakable mirror where she can see it. Pet stores sell them in the bird section. There was a post on here awhile back with a distressed lonely duck who wouldn't eat, and somebody suggested the mirror trick. The duck settled down once she saw another duck with her. It...
And curious, why are you force feeding her peanut butter? Did a vet tell you to do that? I've never heard of giving ducks peanut butter before. I've heard to give them oatmeal or boiled/scrambled eggs, but not peanut butter.
Is your duck alone now or do you still have other ducks? If you have others, is she separated from them or can she see them? If she is a lone duck now, she is likely grieving the loss of her friend. Ducks bond very strongly.