Why didn't Allah answer my prayer to marry her, since she was not destined for me, and is it permissible for me to pray against her after my engagement to another?

Rapid anger, fault-finding in others, and an unforgiving nature disrupt life and make harmony difficult, especially since humans are naturally prone to error. You should not have been quick to anger and exaggerate matters, which led the fiancée to dissolve the engagement, a right she legitimately holds.

If you still desire the girl, and she is of good character and religious devotion, there is no impediment to reconciling with her through her father or someone who has influence over her. Do not heed her mother's advice to take her clothes and go with them to soothsayers; this is an act of disobedience.

As for supplicating against the girl, it is not permissible because she has not wronged you. This is the reason why your supplication was not answered. This is out of God's kindness to His servants, as He answers the oppressed but does not answer the oppressor.