Focus is much like a muscle. It takes practice to stay focused. Exercise your focus muscle through practice. Focus on your breath as an exercise for example. Feel the cool air coming in your nostrils and then feel the warm air leave your nostrils. Repeat for about 5 minutes to begin. When you notice you mind has snuck in a thought other than the cool and warm air, release the thought and go back to focusing on your breathing. The more you practice, the easier it gets. Do this exercise for a week. You will see results and if you see results, why not continue the practice.