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 is His slave and Messenger.
You must repel these whispers, not care about them, and not pay any attention to them.
Praise be to Allah, you are upon much goodness, and you are a Muslim, and you do not go out of the fold of Islam because of these whispers.
As for the proof of the truthfulness of the Quran and that it is from Allah, then this is very evident. The Prophet challenged the entire world, the humans as well as the Jinn to bring the like of what he had brought to them [i.e. the Quran], but they were all unable to do so. Therefore, this is evidence that it is certainly from Allah. Allah Says (what means): {Say, “If mankind and the jinn gathered in order to produce the like of this Quran, they could not produce the like of it, even if they were to each other assistants.”} [Quran 17:88]
Additionally, the Quran informed us about many matters of the Unseen and they occurred exactly as stated in the Quran. How could the Prophet have known this if this had not been from Allah.
The Quran also informed us about stories of former people and the Prophet Muhammad was illiterate, he neither knew how to read nor to write. So how could he have known all of this unless it was a revelation from Allah Almighty.
The Quran guides to good morals and conduct, which is definitely not the words of human beings.
There are many miraculous aspects that prove that the Quran is from Allah, so repel these whispers and stop giving in to them because giving in to them leads you to a great evil.
For more benefit, please refer to Fataawa 8187, 354688, 91361, and 7601.
Allah knows best.