What should an unmarried girl suffering from obsessive-compulsive disorder do if she cannot differentiate between mani (seminal fluid) and madhi (pre-seminal fluid), and is ghusl (ritual bathing) obligatory upon her in this case if she is unable to determine her feeling of pleasure or desire?

Allah the Almighty is merciful to His servants; He does not burden them with hardship. He has legislated for them to ignore and strive against whispers (waswasa). If you doubt whether what has exited from you is semen (maniyy), prostatic fluid (madhiy), or discharge, you may choose between them, and ghusl (ritual bath) is not obligatory upon you. The signs of semen are clear, so if you ascertain that semen has exited with these signs, then ghusl is obligatory; otherwise, it is not. There is no need for excessive fear, and it is most likely that the matter is merely a whisper. Mere arousal does not necessitate the emission of semen and does not obligate ghusl.