But she cautions her heart to tell her right away when its job of forgetting has been carried out, because she is afraid that if it takes the heart too long to forget, it will give her time enough to remember this man one more time...she will not be able to carry out her self-given assignment about the light..."to dim her thoughts." (here I slightly paraphrased, but you get the gist. It is as if she and her heart must do their jobs of forgetting simultaneously or forgetting will not be able to occur