How can I make my heart believe in the khalimah despite my efforts?

Hereunder is the response to your query subject to disclaimer:

Deception of Shaytaan – Doubts – It should be clearly borne in mind that mere doubts and thoughts, no matter how horrific they may be, are not punishable nor is one accountable for them. A person becomes accountable only when one gives evil thoughts and doubts a home. That is to say, the doubts are given impetus and strength by acting upon them. Continue with your Islamic way of life and keep asking Allah for Istiqamah (steadfastness) on Deen.

The fact that you are fighting these doubts is testimony to your Imaan and love for Islam. The Thief (Shaytaan Accursed) only shows up when there is something to steal so you should thank Allah for his Grace and Kindness for Imaan. When you get such thoughts, simple recite Ta’awwudh (A’udhu billahi min ash-shaytaan nir-rajeem) I seek refuge in Allah from Shaytaan the Accursed and Kalimah Tayyibah Laa ilahaa illallah and ignore the thoughts. You need not fight the thoughts, rather you are asking Allah’s protection from Shaytaan.

Do try to make a reasonalble amount of zikr daily. Preferably one Tasbeeh of Istighfaar, i..e recite Astaghfirullah-al-Azeem, Astaghfirullah-al-Azeem ….

one Tasbeeh of the Kalimah and one tasbeeh of Durood Sahreef, i.e. Sallallahu Alan-Nabiyyil-Ummi

Insha-Allah, through the blessings of zikr, Shaitani thoughts will subside. Shaitan cannot attack a heart which is illuminated with the radiance of Allah’s remembrance.

The following article would be very useful Insha-Allah Ta’ala:

10 Ways of Protection from Shaytan
Imam Ibn ul Qayyim al Jawziyyah
Summarized from Bida?i` al-Fawa?id (304-308)

Seeking refuge with Allah from Shaytan. Allah the Most High said, ?And if there comes to you from Satan an evil suggestion, then seek refuge in Allah. Indeed, He is the Hearing, the Knowing.? [41:36]
Recitation of the two soorahs al-Falaq and an-Nas, as they have wondrous effect in seeking refuge with Allah from his evil, weakening Shaytan and protection from him. This is why the Messenger, sallallahu `alayhi wa sallam, said: ?No person seeks refuge with anything like the Mu`awwidhatayn (soorahs al-Falaq and an-Nas)?. [an-Nasaa?i, 5337]
Recitation of Ayat al-Kursi (2:255).
Recitation of soorah al-Baqarah. The Messenger, sallallahu `alayhi wa sallam, said, ?The house in which al-Baqarah is recited is not approached by Shaytan.? [Muslim]
The final part of al-Baqarah. The Messenger, sallallahu `alayhi wa sallam, said, ?Whoever recites the two last verses of al-Baqarah at night they will suffice him.? [Muslim]
Recitation of the beginning of soorah Mu?min (Ghafir), until His saying, ?wa ilayhi-l-maseer? (to Him is the destination). (i.e. ?Ha. Meem. The revelation of the Book is from Allah, the Exalted in Might, the Knowing, the forgiver of sin, acceptor of repentance, severe in punishment, owner of abundance. There is no deity except Him; to Him is the destination.? [40:1-2])
Saying ?la ilaha ill Allah wahdahu la sharika lah, lahul mulku wa lahul hamdu wa huwa `ala kulli shay?in qadir? (there is nothing worthy of worship except Allah, He has no partner, His is the Dominion and Praise, and He is able to do all things) a hundred times.
The most beneficial form of protection from Shaytan: abundance of remembrance of Allah, the Exalted.
Ablution and prayer, and they are from among the greatest means of protection, especially at the time of emergence of anger and desire.
Abstinence from excess speech, food and mixing with people.

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(Mufti) Abdullah Patel
Halal Food Guide