All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
We ask Allah, The Exalted, to safeguard you from this Waswasah (obsessive doubts), and we advise you to disregard it and try hard to overcome it. Please, refer to fatwa 199253 about the useful remedies for Waswasah.
As for the pronouns and the plural and singular forms in supplication, this is a flexible matter and does not require much concern and overthinking. In fact, concentrating on such issues is a consequence of Waswasah. It was reported that the Prophet, sallallahu ‘alayhi wa sallam, used to seek refuge from Satan using the masculine pronoun, and he used to seek refuge from the evil of the self using the singular form at times and the plural at others. It is prescribed for us to seek the forgiveness of Allah for a particular sin or for all sins in general. It was narrated that the Prophet, sallallahu ‘alayhi wa sallam, said in the Farewell Hajj, “...And we seek refuge in Allah from the evils of our souls and our misdeeds.”
Abu Bakr, may Allah be pleased with him, said, “O Messenger of Allah, command me with words that I may say in the morning and evening.” He, sallallahu ‘alayhi wa sallam, said, “Say, ‘Allahumma faatir-as-samaawaati wal-ardh, ‘aalim-al-ghaybi wa-sh-shahaadah, rabba kulli shai’in wa maleekah, ash-hadu alla ilaaha illa ant, a‘oothu bika min sharri nafsi wa min sharri ash-shaytaani wa-shirkih.’ (which means: O Allah, Originator of the heavens and the earth, Knower of the Unseen and the Seen, Lord of everything and its Possessor! I bear witness that there is no god worthy of worship but You, I seek refuge with You from the evil of my soul and from the evil of the devil and his Shirk [polytheism].) Recite this Thikr [expression of remembrance of Allah] in the morning and the evening and when you go to bed.’” [Abu Daawood - At-Tirmithi graded it hasan saheeh (good or sound)]
Abu Hurayrah, may Allah be pleased with him, narrated that the Messenger of Allah, sallallahu ‘alayhi wa sallam, used to say in his prostration, “Allahuma ighfir li thanbi kullahu, diqqahu wa jillahu, wa awwalahu wa akhirahu, wa ‘alaniyatahu wa sirrah.” (which means: O Allah! Forgive all my sins, the small and the great, the first and the last, the open and the secret.)
Abu Moosa Al-Ash‘ari, may Allah be pleased with him, reported that the Prophet, sallallahu ‘alayhi wa sallam, used to supplicate:
“Rabbi ighfir li khatee'ati wa jahli wa israafi fi amri, wa ma Anta a‘lamu bihi minni. Allahumma ighfir li khataayaaya wa ‘amdi wa jahli wa hazli wa kullu thaalika ‘indi. Allahumma ighfir li ma qaddamtu wa ma akhkhartu, wa ma asrartu, wa ma a‘lantu, wa ma Anta a‘lamu bihi minni. Anta Al-Muqaddimu wa Anta Al-Mu'akhkhiru wa Anta ‘ala kulli shai'in Qadeer.” (which means: O Allah! Forgive my errors, ignorance, and immoderation in my affairs. You are better aware of my faults than me. O Allah! Forgive my sins, my faults which I committed in seriousness or in fun deliberately or inadvertently. O Allah! Grant me pardon for all those sins which I committed in the past and which I may commit in the future, which I committed in privacy or in public, and all those sins of which You are better aware than me. You are The Expediter, and You are The Delayer, and You are over all things Omnipotent.)
Allah knows best.