Mamyalynne is right. 'Keep your shoulders down' is the big thing with breathing. I have suffered similarly - my GP said to relax - if I passed out then body would take over and I'd breathe naturally. I now suffer from asthma and have re-discovered how anxiety about breathing can hinder it.
Relaxing is so hard, but, honestly, sitting down, thinking about keeping your shoulders down and breathing into your stomach works. I'm yawning deeply now, just talking about it. It is all to do with mind control - control those shoulders! Good luck! :)