Based on the provided text, what was Radhabai's primary motivation for coming to Shirdi and undertaking her severe fast?

Radhabai's primary motivation, as explained in Chapter 18, was to make the capable Sai Baba her Guru and receive a personal instruction or secret mantra directly from his mouth. After hearing of his fame, she came to Shirdi and developed a deep faith at his feet. A longing arose in her heart to have him as her Guru, believing his instruction would help her achieve her spiritual goal. Her resolve was so firm that she decided she would not accept a mantra from anyone else, stating it would be 'impure'. This singular focus led her to undertake a severe fast, vowing not to eat or drink until Baba personally made her a 'recipient of His grace'.