I have been suffering from the same problem that all you have been describing.
I find that the tight chest and awareness of every breath is often triggered by something first such as anxiety over going somewhere or doing something but then even when i am totally relaxed it is still there and can sometimes last for months.
It is often worse at night also but i think that this is becuase i am in bed alone with my thoughts so focus more on my breathing thus tightening up etc etc creating a vicious circle. I dread going to bed for this reason so havent been sleeping well.
I am aware it is all anxiety based as i have been suffering with this on and off for at least five years and im still alive and in perfectly good health everywhere else.
I, like a few of you have said often have serious panic attacks due to this feeling and awareness of my every breath and it has made me deeply depressed and put alot of pressure on my relationship.
I think the reason the sensation and anxiety doesnt go away is becuase i always dwell on the worst case scenarios and assume this shallow breathing will kill me!! obviously it hasnt and wont but its so hard for the logical part of your brain to override the irrational part that says "panic panic your dying!!!"
I feel better knowing other people suffer from this and im not alone. i suggest we all just try and take it easy and remember we are all here alive and kicking so it ISNT SERIOUS OR LIFE THREATENING just very very very very unpleasant.
we can all beat it....just try to keep calm and tell yourself you are going to be ok.
good luck everyone.....breathe easy xxxx