What is the ruling on setting conditions in an oath concerning a forbidden act, and what should I do after repeatedly breaking this oath, given my inability to fulfill the conditions?

The condition added to an oath is "nathr al-lajāj" (a vow made in anger or a fit of obstinacy), by which its maker intends to urge himself or restrain himself from something. If he fulfills it, there is no problem, and this is the original obligation if it is an act of obedience.

However, if he breaks it, he is given the choice between expiating for his oath or fulfilling the vow. Ibn Qudamah said: "If a vow is made in the form of an oath... then this is an oath. Its ruling is that he is given the choice between fulfilling what he swore to... or breaking it, in which case he chooses between performing the vowed act and performing an expiation for an oath." This is called nathr al-lajāj wal-ghaḍab (vow of obstinacy and anger), and he is not obliged to fulfill it. This opinion has been narrated from a number of Companions, who cited the saying of the Messenger of Allah, peace and blessings be upon him: "There is no vow in anger, and its expiation is the expiation of an oath."

If the one who swore an oath or made a vow breaks his oath, that oath is dissolved, and he is only obligated to perform one expiation, unless he swore that every time he commits the sin, he will be obligated to do such and such, in which case he is obligated to perform an expiation every time he breaks it. The most important thing is to repent from the mentioned sin, work to avoid it, and turn to Allah in obedience.