I personally think that the best way to start creating a thesis statement that is based on a particular work is to pick a theme of that work and to use that as the basis for a thesis statement. Clearly, in this challenging and impressive work, one key idea is that perseverance. Keller is of course the major example of this, as she had to work so hard to become accepted in society and fashion her own niche in the world. Once she had overcome the many profound obstacles that caused her such difficulty, she showed considerable energy and perseverance in pursuing her goals. Throughout her life, she shows considerable determination. Consider what she says when she heard about a Norwegian girl who learned how to speak:

Mrs. Lamson had scarcely finished telling me about this girl’s success before I was on fore with eagerness. I resolved that I, too, would learn to speak.

In spite of the difficulties she faced in the logistics of learning to speak, Keller kept on at it until she achieved her objective. Therefore, this would be an excellent basis for a thesis statement. Perhaps you could use the following statement:

In The Story of My Life, the character of Keller shows the value of perseverance in being able to achieve any goals you set yourself, no matter how hard.

Such a thesis statement would allow you to discuss the character of Keller and how she displayed perseverance in the story of her life.