Dear questioner,
Thank you for your important question.
May Allah, Most High, reward you for your desire to increase in knowledge.
Yes, it is absolutely ok to perform your voluntary remembrance silently in your heart.
Do not consider this silent form of remembrance as inferior to vocal remembrance. The Prophet Muhammad (Allah bless him and give him peace) said:
‘The best remembrance is that which is done quietly, and the best provision (livelihood) is that which is sufficient.’ (Ibn Abi Shayba, al-Musannaf)
Shaykh Amin al Kurdi says: ”Know that there are two kinds of remembrance; of the heart and the tongue. Each has its legal proofs in the Qur’an and the Sunna…. By the remembrance of the heart, nothing distracts one from his remembrance.
(Amin al Kurdi, Tanwir al-Qulub)
As for the difficulty in pronouncing words, please recite the verse often; “Lord, lift my heart and ease my task for me. Untie my tongue so that they may understand my words.”
رَبِّ اشْرَحْ لِي صَدْرِي وَيَسِّرْ لِي أَمْرِي وَاحْلُلْ عُقْدَةً مِّن لِّسَانِي يَفْقَهُوا قَوْلِي
(Quran, 20:25-28)
May Allah, Most High, alleviate your difficulties and raise your rank in paradise.
And Allah alone knows best,
[Ustadh] Omar Popal
Checked and Approved by Shaykh Faraz Rabbani