What should I do, given that my fiancée's family broke their promise of marriage after a year and a half, claiming she is too young, even though she loves me and her father agreed to the marriage when she turns 18?

If you love a girl and her family refuses to marry her to you, try proposing to her again through mediation. If they insist on refusing, then seek someone else who is characterized by religiousness and good character, for perhaps the good lies therein. Note that their refusal after having initially accepted the suitor is not forbidden, but it is disliked if there is no reason for it.