Think about what YOU believe is causing your inability to focus. If it's stress (which includes "worry," etc), nutritional, hormonal, or something else. If one of these recommendations resonates with you, that's where I would start!
Stress: Inhaling authentic, unadulterated essential oils (not the typical ones you find in the marketplace that are super cheap) can actually facilitate a response in the brain that allows you to be more calm & more focused. The oils I have found to help the most people with lots of stress are: Stress Away, Peace & Calming roll-on, Sacred Frankincense (especially good for spiritual connection), Valor roll-on, Lavender, Grounding & Envision (especially good for meditation & envisioning). There are lots of other possibilities, but these are the ones I've seen most people gravitate to.
Simply putting a little bit (a drop or two from the bottle, or a swipe from the roll-on) in the palm of your hands and cupping them over the nose while you inhale deeply through the nose for a couple of minutes can totally help you feel more centered & focused. It can even bring down the stress levels in your body - (think cortisol) that get ramped up in times of stress!