I have no "reserve" for holding my breath

I am a former smoker and Pharmaceutical industry person. I have COPD which limits the elasticity of lung tissue. I trying to build myself back up by swimming a half or to walk a mile a day in the pool. I have just discovered that i need to work even harder to go though the lung operation and I am trying to make my body stronger by going to rehab and swimming classes every day.
The problem is that I have no "reserve" for holding my breath for any length of time. I've tried meditation, deep breathing prior to swimming and just about anything else that I can think of including the Power Lung. After less than 10 seconds, I'm out and it's difficult to build up a cushion again.
Is there any workable exercises or tips that I can use to increase my lung power? Any help would be appreciated. If this has been covered before, I apologize.