Is his decision to end the relationship with the girl he promised to marry and to seek another wife to ensure repentance and chastity considered correct, or is it deemed a breach of promise, especially since he cannot marry her under his current circumstances?

You must repent sincerely and sever all contact with the girl through whom Satan almost tempted you. Then, if you wish to marry her, you may do so if she is religious and of good character; otherwise, seek someone else. Fulfilling a promise of marriage is not obligatory according to the majority of scholars. It is preferable to marry quickly to protect and fortify oneself, as the Prophet, peace and blessings be upon him, guided.