What should a servant do if his heart deviates, and are there previous examples of people whose hearts Allah caused to deviate and then guided them?

Hearts are between two of the fingers of the Most Merciful; He turns them as He wills. He alone is distinct in guiding and leading astray. So, whoever feels his heart deviating after guidance must repent to his Lord and turn to Him, for the door of repentance is open as long as the soul has not reached the throat. There is no rectitude for the heart except by turning to Allah the Almighty, prostrating oneself before Him, humbling oneself to Him, and asking Him for steadfastness and assistance. The Prophet, peace and blessings be upon him, used to supplicate: "O Turner of hearts, make my heart firm upon Your religion." And those firmly rooted in knowledge say: "Our Lord, let not our hearts deviate after You have guided us, and grant us from Yourself mercy. Indeed, You are the Bestower." The servant must adhere to these supplications, be sincere in approaching his Lord, and strive to take the means for the rectitude and uprightness of his heart.