Why did Helen have to sometimes repeat words & phrases for hours at a time?

From the text:

In reading my teacher's lips I was wholly dependent on my fingers: I had to use the sense of touch in catching the vibrations of the throat, the movements of the mouth and the expression of the face; and often this sense was at fault. In such cases I was forced to repeat the words or sentences, sometimes for hours, until I felt the proper ring in my own voice. My work was practice, practice, practice. Discouragement and weariness cast me down frequently; but the next moment the thought that I should soon be at home and show my loved ones what I had accomplished, spurred me on, and I eagerly looked forward to their pleasure in my achievement.

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