Any good source of energy. I need to stay focused at work every day.
My thought first is to get 6-8 hrs of sleep. Restful sleep! Get 30 mins of walking each day! Next, is learn to meditate. That helps to focus your brain. And be sure that you get enough water intake! Then keep a journal and get your worries down on paper, they can become clearer, when viewed, instead of rambling around in our head! Good luck!
A healthy consistent breakfast before work including adequate vegetables, proteins, healthy fats, and medium-chain triglycerides. Pair this with consistent, adequate amounts of restful sleep every night. Avoid alcohol, tobacco, caffeine, sugar, and other stimulants; they just put you on an energy roller coaster that is hard to get off.
You would be amazed how much focus you have when you aren't hungry or having blood-sugar spikes and crashes, and how much energy someone your age can produce naturally just by taking good care of yourself.