Is it permissible to imagine a girl who does not exist to avoid committing to a real girl?

A Muslim should not surrender his heart and mind to corrupt fantasies, for they are a mirage and an illusion that cause weariness of the heart and anxiety of the soul. These thoughts corrupt the heart, and if one indulges in them, they multiply and are fostered by Satan until they take over the heart, and then manifest through the limbs. Your belief that this action will protect you from falling into a relationship with a foreign girl is mistaken; rather, it is one of the greatest causes that lead to infatuation and forbidden relationships. Ibn al-Qayyim says: "Repel the fleeting thought, for if you do not, it becomes an idea. Repel the idea, for if you do not, it becomes a desire. Fight it, for if you do not, it becomes a resolution and determination. If you do not repel it, it becomes an action. If you do not counteract it with its opposite, it becomes a habit, and it will be difficult for you to move away from it." He also says: "The first thing that strikes the heart is the fleeting thought. If it is repelled, one is relieved from what follows. If it is not repelled, it strengthens and becomes a whisper... If it is not repelled, it strengthens and becomes a desire... If it is not treated, it becomes a will... If it is not treated, it becomes a resolution. And once it reaches this state, it cannot be repelled, and action inevitably accompanies it." These bad thoughts and ideas are dangerous and a strong pretext for falling into the forbidden. It is incumbent upon the sensible person to repel them from himself by every means possible.