An inability to concentrate is a frequent symptom of severe ongoing anxiety disorders. The reason for this is logical if you consider the role of the survival instinct.
When it gives a warning and we react with raised negative arousal, it then locks our attention on the source of the danger so that we can take action to save ourselves. Should we become momentarily distracted at this time it could be fatal - or so the survival instinct believes.
Most of the time, when the survival instinct is just monitoring, we are free to focus our attention on whatever we please.
When the survival instinct gets a danger message it will take over and lock our focus onto the danger. When our focus is locked onto a passing danger, such as a momentary threat from a spider for a spider phobic, we can't easily concentrate on anything else, and we aren't meant to.
Usually, once the danger is past, the survival instinct goes back into watching mode and we are able to concentrate on other things.
If, however, we are suffering from a more severe anxiety disorder, such as PTSD or an acute anxiety state, then the arousal level remains high and the ability to concentrate on anything to any degree is impaired.
The ability to concentrate returns as the arousal level is reduced on a regular basis.
I find, however, that people who are already suffering high levels of anxiety, worry that they can't concentrate. This is just making things worse.
Another problem is that, because they can't really concentrate as well as they'd like, they don't try to concentrate at all. This is a mistake. You can help your anxiety levels to return to normal if you allow yourself some of the little everyday pleasures that interest you. This may be doing a little but of gardening or flicking through a magazine.
The trick here is not to expect too much of yourself.
True you won't be able to concentrate as well as you can at your best, but just enjoy what little you ARE able to do. As the anxiety levels reduce the ability to concentrate will return.
Nothing helps anxiety to reduce faster than doing things or getting engrossed in things which give pleasure. You may not be able to keep this up for as long as you can when you are well, but that doesn't matter. It's about doing what you CAN do however little, rather than what you can't.
As the old saying goes:
"A little bit of what you fancy does you good."