Does recalling details of a previous forbidden relationship that leads to secretions and a sense of pleasure constitute a sin, and does it necessitate Ghusl or invalidate the fast, and how can one get rid of these feelings?

Islam has forbidden friendship between a man and a woman who are strangers to each other, except within the framework of a legitimate marriage, due to the severe consequences that can result from it.

Imagining haram [forbidden things] and indulging in them is impermissible. If such thoughts come to your mind, you must reject them, seek forgiveness, change your line of thinking, or occupy yourself with an act of worship or a beneficial habit. This is because idleness and solitude can lead to thinking about such matters.

What is discharged upon thinking about these matters is impure (najis) and must be purified from the body and clothes. If it is madhiy (pre-seminal fluid—a thin, white discharge), then wudu' (ablution) is sufficient. If it is maniy (seminal fluid—a thin, yellow discharge), then a full bath (ghusl) is obligatory, and it invalidates the fast.