Guidance for one who feels fear and anxiety due to committing sins

I have a letter from Shaykh Abdulaziz that describes some psychological states that affect some of the youth. This young man says: I am a seventeen-year-old boy who, praise be to Allah, performs what Allah, Mighty and Majestic, has made obligatory upon His servants from the pillars of Islam and its duties, and I strive to put my soul in the best possible state. However, I have a feeling in my heart through which I sense that I am committing a grave sin that will also lead me to the punishment and anger of Allah, and I have committed it without realizing it. I feel that I am like the slave of the Prophet (peace and blessings be upon him) who was struck by an arrow from the disbelievers, and after Khaybar the Muslims said, "May he have Paradise," so the Prophet (peace and blessings be upon him) said: .
The summary of the matter is that I am in sin and I do not know it. I ask you to answer me regarding this feeling: is it a sign of good and mindfulness of Allah, or a sign of something else? And I ask for your comment on this. May Allah reward you with good.
Answer: This is among the signs of your intense fear of Allah, Mighty and Majestic, and your exaltation of His sanctities. So you are upon good, if Allah wills. And you must stay away from this fear which has no basis, because it is from Shaytan to tire you, make you anxious, and narrow your life. So know that it is from the enemy of Allah when he saw from you the love for good, and saw from you zeal for Allah, and saw from you hastening to good deeds, he wanted to tire you. So disobey him and stay away from what he wanted from you, and be reassured by your Lord. And know that repentance is sufficient, even if the sin is greater than all great things, for the repentance of Allah is above that. And there is no sin greater than shirk (associating partners with Allah), and whenever the Muslim repents, Allah accepts his repentance and forgives him.
So you must repent from what you have known you did, and after repentance everything ends. And it is not permissible for you to have these doubts or to obey the enemy of Allah in this shame or in this fear which may harm you. But know that, by the praise of Allah, you have achieved a great victory through sincere and heartfelt repentance, as the Master, Glorified and Exalted is He, said: [Ta-Ha: 82]. And there is a greater verse in meaning, which is: whenever the slave repents and follows his repentance with faith and righteous deeds, Allah replaces that evil with a good deed, placing a good deed in place of every evil deed, as He said, Glorified is He, in Surat al-Furqan: يُضَاعَفْ لَهُ الْعَذَابُ يَوْمَ الْقِيَامَةِ وَيَخْلُدْ فِيهِ مُهَانًا ۝ إِلَّـا مَن تَابَ وَآمَنَ وَعَمِلَ صَالِحًا فَأُوْلَـئِكَ يُبَدِّلُ اللّهُ سَيِّئَاتِهِمْ حَسَنَاتٍ وَكَانَ اللّهُ غَفُورًا رَّحِيمًا [Al-Furqan: 68-70]. So He, Glorified and Exalted is He, informed that He replaces the evil deeds of these people with good deeds because of their sincere repentance, their faith, and their righteous deeds.
So you, by mentioning the sin you referred to and repenting from it, and following what happened from you with righteous deeds, and with faith and belief, and desire for what is with Allah, Allah, Glorified and Exalted is He, will replace for you in place of that evil deed a good deed. And so with all the evil deeds from which the slave repents and follows with faith and righteous deeds, Allah will replace them for him with good deeds, Glorified and Exalted is He, out of His favor and kindness. Yes.