You might want to look into whether something else is going on medically as to your lung capacity, as in air pollution/alergies to the air gunk.

I discovered that I had an alergy problem that was constricting my air intake. I also had a Rhino-vastitis. That's where my nose dripped more than the average rider...as in before, during, and after a ride. It got to the point that I could not suck my Camelback during a ride from the air born-alergies/runny nose.
I had to do something. My doctor put me on an inhaler, I began spin classes 3 times a week and since then, my riding endurance and lung capacity has surpassed my wildest. I can not only breathe again, but have more lung power than ever!

I'm now a "climber" on my mountain bike and I attack up hill with reckless abandon ever chance I get...a real change for me. Now I have to work on the down hill stuff...a few too many falls has been making me rather "shy" all of the sudden and I'm not taking the stuff I use to...that has nothing to do with lung capacity, but, fear/anxity drains you energy regardless the challenge.

Add some spin classes for endurence...most are 45 mintues and you'll find other cyclist there working on themselves. Good luck.
Terry