I think that Lady Macbeth's own mental deteiroration as the play progresses reflects that she is not as strong as she or the audience would like to believe. I wonder about the use of the word "strong." It seems to me that she is strong enough to be human, more human than her husband. Yet, she does demonstrate that there is a great deal of change in her as she was from the original insistence that her husband kill Duncan.
The sleepwalking scene is probably the best example of her breakdown. It reflects how much she has changed from the original state where she was demanding that Macbeth kill Duncan. The psychological implications of this scene are profound as it reflects a great deal of discomfort in what has been. It also establishes the basic idea that she, as a character, has changed. Perhaps, her commitment to the ends of murder and power to which she aligned herself so strongly has wavered and this can be read as being in not so strong of support of such ends. This is also scene in the last Act of the play, when she seeks to remove "the spot," that stain of murder and foul play. When humanized into "her own guilt ridden madness," I am not sure if this reflects weakness, as much as a belief that the moral path pursued was not a right or accurate one. In this, she shows a different side to her character. She might be "weak," or "not as strong." Yet, it is only showing that her commitment to evil ends has waned, cast in stark opposition to her husband.