How can one mend the heart of a girl who waited and was harmed by the refusal to marry her due to parental disapproval, and is the suitor sinful after his parents had previously promised him the marriage?

If the girl is religious and of good character, the parents should not object to marrying her, and efforts to convince them should continue. However, if they insist on refusing and you fear that they might be harmed, then you must obey them and abandon marrying her, because disobeying them is not permissible if they would suffer significant harm. As for the promise, fulfilling it is not obligatory according to the majority of scholars, and you have been prevented from fulfilling it by a valid impediment.