Hello dear,
The reason for excessive mental fatigue will need a detailed assessment with regards to your physical and mental health. This assessment is likely to include a detailed psychological and medical evaluation including some investigations like
thyroid, anemia,
vitamin B 12, folic acid,
vitamin D etc. Hence please seek a consultation for detailed assessment. Taking any medication without consultation is not advisable and you should avoid that..
Hope this answers your queries. Best wishes for your health and future..