Is thinking about a young man, known to be attached to another girl who is not veiled, considered forbidden, and what are the ways to deal with this situation?

There is no sin in love as long as it does not lead to what is forbidden, because love is a matter of the heart that a person does not control. Therefore, merely thinking about the young man is not considered forbidden, but it should be repelled. To treat this, Ibn al-Qayyim mentions several methods: making oneself despair of the beloved, curing the mind by realizing that attachment to something unattainable is madness, remembering the evils this desire brings and the benefits it prevents, recalling the beloved's flaws and shortcomings, and finally, sincere recourse to Allah through supplication and humility. This must be accompanied by chastity, keeping the matter secret, and not defaming the beloved.