What is the ruling on someone who vowed to fast on Mondays and Thursdays if her mother recovered, then found it difficult to fulfill her vow due to work circumstances and other reasons?

Whoever vows an act of obedience, its fulfillment becomes obligatory upon him, for the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." If he is unable to fulfill it, then he must offer expiation for an oath. The hardship of work does not permit breaking the fast unless it reaches a level where fasting becomes impossible. Regarding one who breaks a vowed fast due to a legitimate excuse (like menstruation), there is disagreement among scholars. As for one who intentionally breaks the fast without an excuse, the preponderant opinion is that making up the day is obligatory.