What should one do who swore by Allah not to return to doing something in order to be pardoned for a transgression, but then became unable to adhere to his oath due to its difficulty?

The basic principle regarding laws organizing people's lives that do not contradict Islamic law is that they must be followed. It is not permissible for a Muslim to violate them, due to the corruption and self-abasement that would result. If a person does not adhere to a commitment they swore to uphold, then they must offer expiation for breaking an oath: feeding ten poor persons, or clothing them, or freeing a slave. If they cannot find these, then fasting for three days.