What should I do about my feeling that I will fall during prayer, knowing that I do not feel this way after finishing it, and that this condition does not always happen to me?

Satan's deception of a person regarding their prayer involves instilling whisperings (waswas) and distractions, which may cause them to lose track of how many [rak'ahs] they have prayed. If one experiences this, they should seek refuge with Allah from the accursed Satan and spit lightly to their left three times, as 'Uthman ibn Abi al-'As did, guided by the Prophet, peace and blessings be upon him. It is also recommended to recite the Quran with reflection and to maintain the morning and evening remembrances (adhkar) to strengthen the heart against Satan.