The Shari'ah Treatment for the Condition of Distress and Doubts

Q: A questioner writes in a long letter, the summary of which is:
I am currently living in a state of misery and suffering. By the grace of Allah, I lived in happiness and tranquility until four years ago. I was turning to Allah, counting everything as for the sake of Allah, standing in prayer, fasting, and calling to Allah. My heart was breaking with jealousy for the Muslims and their conditions, to the point that I participated in the Afghan jihad. I made a covenant with Allah there not to return until victory was achieved, but I weakened and returned after two months due to my father's insistence on my return.
Then my life changed until I was afflicted with doubts about the existence of Allah, the truth of the Qur'an, and the Messenger. I defended that [truth], and I researched tapes and books of Shaykh al-Zindani, and the truth became clear to me like the sun. However, despite that, whispers and doubts return to me.
And the greatest calamity is that I no longer find a place in my heart for the fear of my Lord! And I seek forgiveness from Allah. This condition has been with me for about a year and a half, and I am suffering from it. I have not left anything that would lead me to certainty and drive away those doubts, but to no avail. Even if the truth settles in my heart for a little while, I do not remain but return to what I was before. I used to fast the white days and perform the late-night prayer, and I am still doing so now, but I do not feel any pleasure in it.
I sold my car and donated its value in the cause of Allah, hoping that Allah would return me to Him with a beautiful return. Every day I find myself in a worse state than the previous day.
I turned to the Qur'an, reading a part of it every day with reflection, but I do not find certainty or any effect in my heart except a little. I find a terrible hardness in my heart and a veil and a covering over the truth. I fear the anger of Allah for what I have committed. I do not forget to mention that for a period I have been donating about a quarter of my salary in the cause of Allah and sponsoring orphans, and I am still doing so. I do not take heed from the admonitions as I used to. I accuse myself of open sinfulness and lewdness, and sometimes of disbelief. I have not left any means or admonition without trying it, but the doubts, suspicions, and whispers are destroying me, and I have not been able to get rid of them.
So, my Shaykh and my beloved: You are the only hope after Allah the Exalted in this world, and I will not complain of my condition to anyone after you, no matter how far it goes. Is there a solution and a treatment for what I am suffering from, as I have mentioned? I will seek help from Allah the Exalted and carry out your order, if Allah wills. I hope that you will make du'a for me in the darkness of the night that my Lord may reach me with His mercy and return me to Him with a beautiful return. May Allah reward you with the best reward. And peace be upon you and the mercy of Allah and His blessings.
Your suffering son - from Khamis Mushait.

A: In the name of Allah, and may the prayers and peace be upon the Messenger of Allah, and upon you be peace and the mercy of Allah and His blessings.
As for what follows: I have read all of your letter, and I was greatly distressed by much of the doubt and whispers that have befallen you. I ask Allah to grant you guidance and return to the truth, and to fill your heart with correct faith, and to bestow upon you sincere repentance, and to protect you from the stings of Satan. Indeed, He is Generous and Bountiful.
Some of the Companions experienced something similar to what happened to you, regarding doubts about Allah the Exalted. So the Prophet (peace and blessings be upon him) advised them that whoever feels something of that should say: "I believe in Allah and His Messengers," and that he should seek refuge in Allah and stop. So I advise you with what the Prophet (peace and blessings be upon him) advised his Companions, and that you say these words when a thought, meaning a doubt, occurs:
I advise you to persist in that and repeat it at every bad thought. I also advise you not to despair of the mercy of Allah and not to lose hope. You must increase in reading the Noble Qur'an and reflecting on its meanings, and supplicating to Allah with sincerity, desire, and fear that He will guide you to the truth and remove these whispers from you.
Increase in that during prostration, in the last part of the night, and between the adhan and the iqamah. Have a good opinion of Allah, for He is the One who said the Exalted, as narrated by His Prophet Muhammad (peace and blessings be upon him): And in the other wording: And you must keep the company of the righteous, and beware of the company of the wicked.
May Allah guide us and you to what He is pleased with, and lead us and you upon His straight path. And peace be upon you and the mercy of Allah and His blessings.