That you feel difficult to focus on things refers that when you see things, you see double images. It may indicate that your eyes are tired, sometimes I also find it difficult to focus on things when my eyes are fatigue. Under this case, I would blink my eyes and have a rest. Or you may get double vision, which can be caused by the eye diseases or function disorder of extraocular muscles, or there is something wrong with the optic nerve, even the optic nerve center in the brain. I advise you to go to hospital for an overall ocular examination and a magnetic resonance examination of skull and hypophysis.
Your eyes may not focus based on a number of reasons. One possible answer is you have myopia, thus your muscle or pupil has not responded to distance as you look around. Another cause is you get eyestrain, especially when you staring on a computer or on any other object for a prolonged period, your eyes muscle are fatigue so that you vision could not focus any more. On the other side, if you are glasses wearer, maybe the degree of your glasses is not suitable for your requirement. But I highly recommend you to go to see an eye doctor to check it.
When you feel difficult for your eyes to get focus on something, you'd better go to the hospital to have a check. You may get the eye twitching which will cause your eyeballs to be twitching with no control. It is the bacterium that causes it. You should use the anti-inflammation eye drops to release the symptom.