What is the religious ruling regarding my situation, and is there a solution to the problem of sexual thoughts that come to me after finishing the 'Isha prayer, which cause me to delay and combine prayers, knowing that these thoughts are a type of obsessive-compulsive disorder and not actual desires?

To get rid of obsessive thoughts (waswas), maintain your prayers at their appointed times, and increase your remembrance of Allah (dhikr) and recitation of the Quran. Your studies and responsibilities do not justify delaying prayer or combining them; rather, you must repent and promptly perform prayers at their appointed times. Mere obsessive thoughts do not necessitate ghusl (ritual bath). If something is discharged, and it is madhy (pre-seminal fluid), then only purification and ablution (wudu) are required. If it is maniy (seminal fluid), then ghusl is obligatory. You are not sinful for these obsessive thoughts as long as you do not speak about them or act upon them, but strive to rid yourself of them.