Annie's past provides her with the strongest motivation she has to help Helen. Throughout the play, voices are heard offstage that represent the people and events of the past that still haunt Annie. The strongest voice belongs to her brother, Jimmie. Annie blames herself for his death and can't let go of her guilt until she's able to help Helen at the end of the play. When Helen finally understands the connection between the hand symbol and the actual object, Annie realizes she has accomplished with Helen what she wasn't able to accomplish with her brother. She then promises Helen she will love her "forever and ever".
I think the guilt at loosing her brother is high motivation for helping Helen. There are specific times when Annie is transported out of the 'now' back to the moments of Jimmie's death - she is determined not to let another child be lost.
I also think that Annie is motivated by her own salvation, in a sort of 'pay it forward' kind of manner. She has been redeemed and become a useful member of society (when others thought she was lost) and wants to extend that to Helen.