Too many mental notes make for a cluttered mind. All of that unfinished business can sap your mental energy. Put whatever's on your mind on paper or capture it digitally. Think of it as off-site storage.
Train your brain. Any skill worth having requires practice. Learning to focus is no different. Invest time in mastering attention training or meditation. Both are great ways to practice taming distractions and improving focus... Consult nearest physician they will do the needful.