You could try using Valerian root extract (usually supplied as a gel capsule) which reduces anxiety and stress (this sounds like what might be your problem). I find it makes me calm and allows me to concentrate on the issues at hand.

One thing that also helps is to simply put down the book, find a quiet corner where you won't be disturbed. Sit quietly, and concentrate ONLY on slow, shallow breathing for a few minutes. Try to avoid thinking about "everything", just think only about each breath as it comes in, hold for a few seconds, and then the breath going out (hold and repeat). The idea is to try and increase the amount of carbon dioxide in your lungs, not reduce it (this happens with rapid, panicky deep breathing). Just think only about he breathing "I'm breathing in now, breathing in, holding it, holding, breathing out,..." etc. Sounds cheesy - but it works.

Alternatively, and this sounds silly, get hold of a thick brown paper bag and breath in and out of it for a few minutes which will help calm breathing and help you focus.