What was the core motivation and belief that drove the elderly Deshmukhin to undertake a fast unto death?

The elderly Deshmukhin's core motivation was a profound and unshakeable belief that she must receive a sacred instruction, a 'secret mantra,' directly from the mouth of Sai Baba himself. As Chapter 18 explains, she considered a mantra from any other source to be 'impure.' This conviction was so absolute that she resolved to fast unto death in a practice called 'Prayopavesha,' believing that her pilgrimage to Shirdi would be meaningless without this personal bestowal of grace. Her faith was described as 'extreme' and 'solid,' compelling her to renounce food and water in a desperate, stubborn plea for Baba's direct intervention.