Will my marriage be halal (permissible) or batil (void) if I marry the girl I love, after having decided to stay away from her and fall out with her family because I could not declare my love for her, knowing that we still harbor strong feelings of love for each other?

If you wish to marry a righteous woman of good character, propose to her guardian. If it is destined for you, Allah will facilitate it. If the engagement and marriage contract are concluded with the guardian's permission and the presence of two witnesses, the marriage is valid. However, if she is already engaged to another man and has accepted him, it is not permissible for you to propose to her, based on the Prophet's (peace be upon him) saying: "A Muslim is the brother of a Muslim. It is not lawful for him to outbid his brother's transaction, nor to propose to a woman engaged to his brother until the latter gives up." If you marry her while she is engaged, the marriage is valid, but sinful.