In "The Miracle Worker," why does Annie say that she will never love again and what changes her mind?

When Annie Sullivan was young, she lived in an almshouse with her younger brother, Jimmie.  They were orphans and only had each other.  When Jimmie died, Annie was devastated.  She vowed to never love anyone again:

Well, you say with love so easy, and I haven’t loved a soul since and I never will (The Miracle Worker, Act I Scene 4).

Annie arrived in Alabama to help Helen and become her teacher.  Though Annie tried to keep her distance, she came to love Helen.  At one point, Annie told Helen's parents that she did not love Helen.  Despite this, Annie developed a connection with the child.  She became her companion.  When Helen learned how to communicate, Annie spelled into her hand.  Annie told Helen that she loved her, and would love her forever.  She whispered the words, though Helen could not hear her.  Despite the pain of past loss, Annie was able to love again.