How come whenever I start to think about how I am breathing, I invariably feel like I am not getting enough air, and have to start taking deep breaths?
Because your body is designed so as to let your autonomic nervous system take care of all your breathing needs, by itself, and when you start thinking about what your autonomic nervous system is doing, you’re in effect peering over its shoulder, making sure it’s doing a good job, and like all good professionals, it resents this, and slows down on purpose, just to show you who’s in charge.
Leave it alone, is my advice. You don’t want it to quit on you, do you? 
Yeah, I have the same thing about blinking.
Dammit. Now I’m thinking about blinking.
Thinking about breathing is the root of meditation. When you think about how the air feels through your nose, and how it runs down into the deepest parts of your lungs, it is hard to think about all the irritating little grumblies that sap your brain energy most of the time.
Boy, that was an odd little tangent…
That’s OK, but whatever you do, just don’t become Aware Of Your Skull. OOPS, now I’ve done it…
Aware of your SKULL
http://amasci.com/brain/
Sometime I tell me autonomic system that I’ll take over the breathing for a while, and then I forget to do it. At least that’s what I think is happening.
It’s usually when I wake up on the weekend. I do a big stretch, taking a really deep breath and expanding my ribcage. But after the stretch, when I relax all my muscles, I forget to relax my diaphragm. It’s almost as if it’s locked in place.
After about 10 seconds I realize that there’s something I ought to be doing, something important… DUH! exhale.
Oh well, if I ever die from it, at least I’ll have the satisfaction of stumping the coroner. 