I know this is an old post but I want to see how you been doing?
I would like to suggest a great book to you and I swore my life over this along with some people here who had helped me tremendously. This book is called "Brain Lock" by Jeffrey Schwartz. The book was written for people who deals with OCD but this cover pretty much anything that falls under the "Anxiety Spectrum". If you have time to go to Barnes and Nobles or Borders, get yourself this book, read even the first couple chapters...this will give you a great overview of what anxiety is,how the chemicals in your brain are being affected because of anxiety and how to "re-wired" them back to how it should be through CBT. Its amazing.
LindaRK and Doogle had very good points, but I don't think you understand what I've been trying to tell you all along unless you understand how your anxiety works. Your brain is so "wired up" at this point causing to send you false messages of fear that you are manually breathing, therefore you FEEL you HAVE to manually breath. The problem here is not that you have to manually breath, its the fact you are CONSCIOUS you are manually breathing and you want to get rid of it.
First of all, nobody had died because of the fact they forgot to "breath". (Unless they have an existing condition that is severe, but no, this never happened) Second, like I said your brain is "wired up" regarding this being-conscious-of-my-breathing thing that your brain is now constantly have to focus on this and only this alone. At this point in your "anxiety phase", you have altered the chemical components in your brain,thus resulting to you "thinking" and having symptoms...and the most dreaded thing of all IMO is the negative thoughts/thinking..the what I call "what ifs". These things are caused by anxiety, and what I meant by anxiety is that they are not part of you, its almost like a mind of its own that you can't control...but just have to deal with it at this point. If you at least realized that all your symptoms of fear, negative thoughts, and what have you are "caused by anxiety, and not you" then theres no need for you to analyze it. Analyzing makes this worse... just let it go. Ignore all your negative thoughts, your what ifs, your anxiousness....its not you, its your anxiety. When you realized what Im saying, the more its easier for you to deal with being conscious of your breathing, the easier it is to ignore your "thoughts" about it and next thing you know you are back on track. Like I said before, your brain is "wired-up".... you are at this point where you chemically altered components on your brain (not making this up, its a proven fact anxiety can cause brain chemical alteration) and thats what you "think" and "feel" the way you are at this point. Face the fear, recognize it just being anxiety, ignore, and move on.