Will a person be held accountable for internal struggles with doubts about Islam and faith if these doubts quickly dissipate and are followed by seeking forgiveness? And what is the difference between the whispering of the self (nafs) and the whispering of Satan (shaytan), and what is the way to restrain the self from indulging in evil deeds if remembrance (dhikr) repels Satan?

What you are experiencing of obsessive thoughts is common for repentant individuals and others, and it is a trick of Satan to distract a person from his Lord. The Prophet, peace and blessings be upon him, was asked about such obsessive thoughts and he said: "That is pure faith."

So you are not held accountable for these obsessive thoughts as long as you do not firmly believe in them in your heart and act upon them, because the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven for my Ummah what their hearts whisper to them, as long as they do not act upon it or speak of it."

The self whispers just as Satan whispers. What you mention about the conflict between evil and good thoughts in your heart happens to the believer, and salvation lies in inclining towards the good thought. In the Hadith: "Indeed, Satan has an instigation in the son of Adam, and the angel has an instigation." Satan's instigation is "a promise of evil and a denial of truth," and the angel's instigation is "a promise of good and an affirmation of truth." So, whoever finds the angel's instigation, let him praise Allah, and whoever finds Satan's instigation, let him seek refuge in Allah.