Is a promise of marriage to someone with whom one has no legitimate relationship considered forbidden, especially since she fears marrying someone else?

It is not permissible for a girl to speak with boys except out of necessity and on condition that there is no fear of temptation, for the relationship between a man and a non-mahram woman is not from Islam. Islam has preserved the dignity of women and commanded them to observe the hijab. A Muslim girl must adhere to the hijab, and it is not permissible to obey a creature in disobedience to the Creator. As for the young man who wishes to propose, there is no harm in waiting for him if he is religious and of good character, as long as there is no harm or corruption in the waiting. He must propose to you through your guardian. There is no harm in promising him to wait, and in convincing your family of your desire for him after consultation, seeking guidance through istikharah, and relying on Allah, while noting that the suitor remains a non-mahram to the girl as long as the marriage contract has not been concluded.