Hey there! I have struggled for the past few years on studying and have issues with procrastination. They key to concentrating and doing well in your studies is self discipline and time management. It could help if you take some time to sit down and plan a daily schedule for yourself.

Set a goal for yourself: eg. study for at least 3-4 hours a day, and give yourself a reward for completing that goal, like give yourself 1 hour after studying to do something you like (go for a walk/watch tv/internet). And to keep yourself from being bored you could alternate between two subjects in a day. Wishing you all the best! You need to find a source of motivation to keep you going.