Low mood, attention deficit. Advice,

Hello, i m 29 yrs/ f. Medical graduate, I was having depressed mood since last few years,i can live with that if i m just doing daily routine but it is distrubing when i m under stress,especially exams. I can't study of my post graduate exam which i m trying since last 3 yrs, while studying i feel very sleepy,lack of energy, enthusiasm, i seek professional help, and was prescribed tab sertaline 50 mg od which improve my condition intially around 3months but since couple of months I m facing the same problem,, i have quite my job,, came back to home from hostel,, but still not able to study concentratedly,, i usually tends to forget the things not only the study subject but other things as well,, i cant stick to one decision till my goal is achieved, this anhedonia and lack of concentration is causing a serious on me and my family, my exams is in next month kindly help me with your expart advice, thanks