What should a girl do who has committed herself to continuous months of fasting as a punishment for violating her oath to abstain from excessive food, but is unable to fulfill this fast and does not have money for expiation, and wishes to annul this commitment?

Your oath: "If you violate the diet, I will fast for months" is considered an oath of lajaj (a rash or impulsive oath), and you have the choice between fulfilling it or atoning for it. If you fast, it will suffice for you. If you do not fast, then you must perform an expiation for breaking an oath (kaffarat yamin (definition)). The expiation for breaking an oath is feeding ten needy persons, or clothing them, or emancipating a slave. If one cannot find these, then fasting for three days. Fasting is only permissible if you are unable to feed or clothe. Your statement "without monetary expiation" is considered idle talk, as nothing follows from it. Almighty Allah is the One who legislated the expiation for breaking an oath, and its nature cannot be nullified.