Having difficulty focusing could be a number of things, I've never heard that stress was a major factor.. Unless you're talking about eye stress :]
We use our eyes all day long, the muscles controlling them are bound to get tired.
Most cases of trouble focusing, or having to concentrate on focusing have a convergience insufficency, just meaning you need some help in your glasses to keep your eyes pointed in the right direction.
To those of you who said that your glasses rx are good, and there are no health issues- Did your OD check you for needing any prisim correction? I have 20/20 vision yet I need glasses all the time because of muscle issues.
High blood pressure can affect your vision also, and those with high blood sugar.
This doesnt have to do with not being able to focus, but with blurry vision- dry eyes! keep some re-wetting drops handy. NOT VISINE!