What is the ruling on doubtful impurities, especially prostatic fluid (madhy), when they cause obsessive-compulsive disorder (OCD) that leads to washing everything one touches, such as furniture, floors, and cars? Should these obsessive thoughts be ignored, or should everything be washed?

The waswas (whispering of Satan/obsessive thoughts) has reached a great extent within you, and its cure is to ignore it and pay no attention to it. The basic principle regarding things is purity, and nothing is judged to be impure merely based on doubt. If something becomes impure and then the impurity dries, it does not contaminate what it touches if the latter is dry. Do not wash any of the aforementioned things, nor believe them to be impure, except with absolute certainty. What is discharged after masturbation is usually mani (sperm/seminal fluid), which is pure, and it is not obligatory to wash whatever it touches.