In the name of Allah the most Beneficent and Merciful.
Making a vow/Qasm is a very serious commitment and is not to be taken lightly, especially when it comes to repentance. The Prophet of Allah has stated in a hadith that a person who performs repentance and goes back to that sin is as if he has played a joke with Allah. Therefore one must try their utmost best to keep their promise. However, Shaitaan will also try his hardest so that a person in their weakness will go back to that sin. One should not lose hope and despair but try again as Allah knows your intentions and ask for help from Allah, Insha Allah eventually you will succeed as long as the effort is sincere and true.
So, what is true repentance?
Ibn Hajar mentioned that Qurtubi quoted 23 different definitions of true repentance (al-tawba al-nasuh) in his tafsir. Ibn Hajar mentioned the most important of these:
1. Umar`s words (Allah be pleased with him) that it is, “To sin and then never to return to it.”
2. To hate the sin, and the seek forgiveness for it every time it occurs to one, as Hasan al-Basri (Allah have mercy on him) said.
3. Qatada`s words (Allah have mercy on him), “To be genuine and truthful in one`s repentance,” which is what Imam Bukhari chose as the definition of true repentance in his chapter heading.
4. To have sincerity in one`s repentance.
5. To be concerned about one`s repentance not being accepted.
6. To be such that it does not need another repentance after it.
7. To be made out of fear and hope, and be accompanied by consistency in worship.
8. Like the seventh, but with the added condition that one desert those who assisted one in sin.
9. That one`s sin be between one`s eyes. [That is, one does not forget it.] [Ibn Hajar, Fath al-Bari]
Of course Allah knows best
[Answer provided by: Muhammad Salim Ghisa]