Hello Friend, Good that you are seeking a help here. Such problems are quite common due to over stress and lifestyle disorders, dietary etc.
It's better if you take a professional help, so that you will understand what's causing you the sadness and stress. Also you will assess yourself on your strengths and weaknesses, so that a suitable solution you can follow to bring back your concentration.
Next Steps
Consult a psychologists directly or easily consult online through Practo app chat. Feel free to reach out to me online for further evaluation and recommendation. Be confident. You will be alright soon. All the best.