I was just did a google search on "can't take a deep breath" and found myself here. I am in my mid 30s and have had this problem as long as I can remember (at least pre teens). I have been to several different doctors who have claimed that it is "stress related", "hormonal", and/or possible asthma related. It appears that no body has any real idea on what it is (really, how stressed could I have been at 13?).
I find that during the day if I keep myself busy I can ignore it or at least not let it bother me. My problems arise when I do try and go to sleep. The harder I try to relax by taking a deep breath the more frustrating it becomes. Usually, if I sit up or get out of bed I can "grab" that lung full or yawn which feels great and offers temporary relief.
It is refreshing to hear that there are others out there suffering from this. If anyone can offer advice I am ready, willing and able to try.