Hello Toughling and welcome.
This may or may not help, but I went through something like this where I felt like I had to manually breath...here goes.
When you are breathing and you exhale and begin to inhale, I found that I was only exhaling about half of the air in my lungs. I found that if I set in a chair and exhale normal and then slowly start pulling my stomach in as to force more air from my lungs...that when it was all I could pull in...believe me you won't have to manually inhale, you will take a natural deep breath automatically, oh and inhale using your stomach....not your chest....let your stomach inflate, you will get a lot more air in your lungs. Practice this enough and you will be able to do it without thinking about it so much.
I also found that not only could I pull my stomach in slowly, but that when all the air was out, I actually could take that deep inhale slowly....really helps for relaxing and getting plenty of oxygen in your blood stream....Again, may not work for you, but I found it to work really well.....Best wishes.....WM