I am going through pretty much the same thing right now. I am also a smoker who probably has smoked more than you so I too have been worried about a smoking related illness with the upper pain in my back and the feeling that I can't take a deep breath or yawn. I went to the doctor on a couple of days ago and they showed me that it was anxiety related and put me on Lexapro at night and wellbutrin in the morning. It has helped with my breathing, but i am still having problems with my upper back. I can feel the knotted muscle though, and am sure that as my anxiety goes away, so will my tension that is making my upper back stiff and painful. I hope this helps.