Hearts hardening is described as a process
Two things. The Qur'an itself speaks of hearts hardening over the passage of time, which is a description of something gradual rather than a sudden fall. And there is a piece of research that gives ordinary names to things people otherwise lump together.
Qur’an and Sunnah
۞أَلَمۡ يَأۡنِ لِلَّذِينَ ءَامَنُوٓاْ أَن تَخۡشَعَ قُلُوبُهُمۡ لِذِكۡرِ ٱللَّهِ وَمَا نَزَلَ مِنَ ٱلۡحَقِّ وَلَا يَكُونُواْ كَٱلَّذِينَ أُوتُواْ ٱلۡكِتَٰبَ مِن قَبۡلُ فَطَالَ عَلَيۡهِمُ ٱلۡأَمَدُ فَقَسَتۡ قُلُوبُهُمۡۖ وَكَثِيرࣱ مِّنۡهُمۡ فَٰسِقُونَ
Is it not time for believers to humble their hearts to the remembrance of God and the Truth that has been revealed, and not to be like those who received the Scripture before them, whose time was extended but whose hearts hardened and many of whom were lawbreakers
Qur’an 57:16
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.
What the research says
Exline JJ, Pargament KI, Grubbs JB, Yali AM (2014). The Religious and Spiritual Struggles Scale: development and initial validation. Psychology of Religion and Spirituality, 6(3), 208 to 222 doi
Separates six different kinds of religious and spiritual struggle: with God, with evil, with other people, moral struggle, doubt, and struggle over meaning. Useful because feeling abandoned, doubting that it is true, and feeling morally unworthy are three different states that people describe with the same words. The honest limit is that it was developed and tested largely in American samples that were mostly Christian, so how it behaves among Muslims is an open question rather than a settled one.