Is a girl who does not feel comfortable with her fiancé and has told her family about it considered among the women who "are ungrateful to their husbands/spouses"?

What is meant by "kufran al-ashir" (ingratitude towards the spouse/companion) is the denial of blessings and acts of kindness bestowed upon a person. So, if someone bestows a blessing upon a person and that person denies it and does not reciprocate, perhaps even saying, "I have never seen any good from him/her at all," then this is indeed "kufran al-ashir" and denial of blessings. Breaking off an engagement does not fall under this category.