How can a girl who suffered from "lack of modesty" due to her past and because she used to masturbate through fantasy, rid herself of this feeling after Allah has bestowed guidance upon her?

Shyness (Hayāʼ) is of two types: innate and acquired. To achieve the character of Hayāʼ, you must strengthen your faith in Allah, be mindful of His blessings and His constant watchfulness, remember the grave and the Last Day, ponder His Beautiful Names, emulate good role models, and strive against your soul to abandon anything contrary to Hayāʼ.