Why do I have a hard time concentrating?

Anxiety. Anxiety is also famous for reducing concentration.
Try meditation. Try meditating or start by reading on meditation. It will help you concentrate.
ADD, Depression. Although this behavior suggests attention deficit disorder, it could also be that the issues you are having difficulty with bore you silly and can not hold your interest. If you find that you can not concentrate and focus on something that does interest you, further professional evaluation would be a good idea. Depression or add could both be involved.
Too many distraction. Most of the time , you have too amny things in your mind at once that are fighting for your attention, try to work on focusing on the task at hand and blocking all other thoughts for a limited amount of time; trying to be organized helps a lot since you give your mind the message that there will be a time to attend to everything that needs to get done and it does not need to bombard you .