The ruling on one who makes a covenant with Allah regarding something and then fails to fulfill it

The following is a letter sent by our brother, Marya Hussein Al-Hamran Al-Qahatani. Within his letter, he included seven questions, of which we will select a few:
He asks: I once said, "I covenant with Allah that I will not commit such-and-such an act," but I did not fulfill that covenant. I ask that you advise me and those who are in a similar situation regarding this matter.
Answer: This requires a detailed explanation: If the covenant was regarding a matter that Allah has forbidden for you, and you covenanted with Allah not to return to it and not to approach it, then it is obligatory for you to repent to Allah for returning to it. And whoever repents, Allah accepts his repentance. Repentance consists of three things:
Regret for the past sin, abandoning it, and a sincere resolve not to return to it. If the Muslim does this, Allah accepts his repentance . And the completion of repentance is following it with righteous deeds and steadfastness, as He said : [Ta-Ha:82]. When He mentioned shirk (associating partners with Allah), killing, and zina (unlawful sexual intercourse) in Surat al-Furqan, He said: يُضَاعَفْ لَهُ الْعَذَابُ يَوْمَ الْقِيَامَةِ وَيَخْلُدْ فِيهِ مُهَانًا ۝ إِلَّامَن تَابَ وَآمَنَ وَعَمِلَ عَمَلًا صَالِحًا فَأُوْلَئِك يُبَدِّلُ اللّهُ سَيِّئَاتِهِمْ حَسَنَاتٍ [al-Furqan:68-70]. Thus, He informed that whoever repents and follows his repentance with sincere faith and righteous deeds, He will replace their evil deeds with good ones. This is in addition to the acceptance of the repentance; He accepted it and then made a good deed in place of every evil deed. This is from His grace, generosity, and bounty, Blessed and Exalted is He. And if the sin involves a right belonging to the created beings, such as theft, encroaching on the wealth of some people, or their blood, or their honor, then the owner of the right must be absolved; he must be given his right or he must forgive it. If he permits and forgives, his right is dropped. The same applies to honor, such as backbiting or speaking ill of someone; he must be asked to forgive. If he permits, his right is dropped, unless there is a fear that informing him will lead to harm and no benefit is expected from informing him. In that case, he is not informed, but rather he is prayed for and forgiveness is sought for him. He is mentioned in the gatherings where he was spoken of ill by mentioning the good things known about him that the one who backbit him knows, so that his evil deed is countered with a good deed. This is if it is not possible to inform him of the backbiting that occurred and there is fear of the evil and consequences of doing so. In that case, he is mentioned with the virtues and good deeds known about him in the gatherings and places where he was spoken of ill, so that this serves as a counterbalance to that, along with praying for him and seeking forgiveness for him. And praise be to Allah. Yes.