Trust me, you are not alone. Learning how to let my body breathe on it's own again has been the most challenging thing since my first panic attack (which started w/ the feeling that I couldn't breathe!). My problem was that I thought I needed to breathe more often, so I'd try and take a deep breath and felt like my lungs weren't getting enough air. Really, I was overbreathing (hyperventilating); which caused my heart to beat faster, etc...causing panic. Often at night before falling asleep I would fixate on my breathing, like you, and felt that if I didn't concentrate I wouldn't breathe! So don't feel silly, because I've felt the same thing. This sounds even sillier, but to stop that I concentrate on spelling words (not out loud). So it diverts my attention from breathing....or exhausts my brain from thinking..hehe. Now everyone here thinks I'm really crazy!
A quick fix for feeling short of breath is breathing into a paper bag for a little bit, which blances the oxygen and carbon dioxide in your blood. But, it's important to learn to breathe full and deep from your abdomen, not shallow chest breathing. Look up some abdominal breathing exercises, or buy a book on it. One critical thing is to only breathe through your nose, never through your mouth.
Here's one exercise I do a few times when I feel anxious:
Breathe in slowly and deeply from your abdomen (w/ your nose! not mouth) to a count of 5.
Hold your breath to a count of 5.
Exhale through your nose slowly to a count of 5.
Repeat until relaxed.
Hope this helped a little, and trust me you'll get through this! I've come a long way in a short amount of time.