There could be many factors responsible for your condition May be there any emotional cause which makes you distract.
Or you don't understand the subject now, which causes loss of concentration.
Or you may be suffering from some illness, probably hypothyroidism or anaemia. Get a blood test done for T3 ,T4, TSH and HB.
Take appropriate sleep of about 6-8 hours and eat balanced diet.
Take Homeopathic medicine Kali phos 6x, 4 tab four times daily for a month. Do some exercise and meditation