since your concentration has dropped in the preceding few months, it will need evaluation for any psychological stress, illness or even medical diseases such as anemia, hypothyroidism etc which may present with concentration difficulties. Consult a psychiatrist for a detailed evaluation. Till then what you can do is study in short 30 min capsules, take 10 min break after each capsule, use imaginative ways to memorise things such as building stories around the text. pretending to teach a class while revising your lecture. Also ensure that you get 6-7 hrs of sleep, eat healthy, go for a walk and do pranayam for at least 15 min twice a day. This should help, also consult an eye specialist to get ur vision checked.