What is the ruling on something exiting after finishing the prayer for one who is thinking about his fiancée?

The fluid you find after prayer is madhy (pre-seminal fluid), which is impure (najis) and nullifies wudu' (ablution) by scholarly consensus. If you find madhy on your clothes after prayer, you must repeat your wudu' and prayer. If the discharge of madhy is continuous, and you find a time when nothing is discharged, you should wait for that time. Otherwise, your ruling is like that of a person afflicted with urinary incontinence (sals); you perform wudu' for each prayer after its time has begun, take precautions (e.g., wear a protective pad), and pray the obligatory prayer (fard) and supererogatory prayers (nawafil) with that wudu'. We advise staying away from evil thoughts and occupying oneself with beneficial knowledge and righteous deeds.