Is it permissible for me to shorten and combine prayers, knowing that the distance one way is about 91 kilometers, and I suffer from health conditions and significant physical and mental exertion during my daily studies?

The school of thought of the majority of scholars is that prayer should be shortened for a distance of four "burd," which is 48 miles, and contemporary scholars have estimated it to be approximately 83 kilometers. If the distance you travel between the two cities exceeds this amount, it is sunnah for you to shorten the four-rak'ah prayers and combine the two prayers (Dhuhr with Asr, and Maghrib with Ishaa) until you return. However, if the distance is less than that, you are not permitted to shorten the prayer, but you are allowed to combine the two prayers while performing them fully due to illness. It should be noted that the majority of scholars prohibit a woman from traveling without a mahram, while some allow it if the company is trustworthy. It is advisable to ensure that a mahram accompanies you to avoid disagreement.