Is the failure to facilitate marriage and falling into some sins caused by abandoning a girl after promising to marry her without reason? And what is the correct way to repent, knowing that the girl has married and had children?

You must repent to Allah for overstepping boundaries in your interactions with the foreign girl. You should fulfill your promise as much as possible, for it is highly recommended, and breaking it is disliked. The Quran condemned breaking promises, and the Prophet considered it a characteristic of hypocrites. If you wish to leave the girl, apologize to her. Delay in marriage may be a trial, so you should supplicate frequently and ask [Allah]. Strive to find a righteous woman with the help of trustworthy individuals, and avoid being overly strict with conditions, for perfection belongs to Allah alone.