Why can't I concentrate on the task I've been given instead of being distracted by everything else?

You can start by disconnecting everything else than your current task. Give your task just 25 minutes of pure attention and tell yourself that you'll get distracted only after these 25 minutes. This is a classic technique called pomodoro which increases your attention span. It has worked for me quite a lot in recent past. Slowly and steadily you can increase the time limit as well. Just start from 15 minutes and increase by 5 minutes per day. You'll see tremendous results in just a month of doing your work in such a manner. This way you'll get more things done and hence will be more efficient.