What is the Shar'i ruling and how can one repent after breaking a covenant with Allah to abstain from a certain habit, especially when one has supplicated for happiness in abandoning it and misery in persisting in it?

If masturbation is what is intended, then it is forbidden. And whoever makes a covenant with Allah to abandon a forbidden act must fulfill his covenant, because Allah has urged the fulfillment of covenants in many verses, such as His saying, "And fulfill [every] covenant. Indeed, every covenant will be questioned about" (Al-Isra: 34). Some scholars have stated that whoever makes a covenant with Allah regarding something, it is an oath for which expiation of an oath is required if it is not fulfilled. The door to repentance is open, so if a servant repents sincerely, Allah accepts his repentance, and he should increase in good deeds that expiate sins, as Allah says, "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful'" (Az-Zumar: 53).