What is the reason for the distress a believing woman feels after losing her previous devotion in prayer, and the return of doctrinal whisperings to her, despite her persistence in acts of worship?

This distress is from Satan and his whispers, seeking to distance you from obedience. Its cure lies in striving against the self (mujahadat al-nafs), turning away from these whispers, and exerting effort to achieve humility and devotion (khushu') in worship, for Allah Almighty says: "And those who strive for Us – We will surely guide them to Our ways. And indeed, Allah is with the doers of good." [29:69] Also, seek refuge with Allah from Satan, and strive diligently in supplication and resorting to Allah Almighty.