Get a "Incentive Spirometer" And use it. I could only do a 450 at first now I am up above 1500. The hospital or your doctor should have given you one. My daughter is a hospital supervisor at a different hospital. One call and She made sure I left with one.
Good to hear you are doing much better. Those spirometers work. You wouldn't think that exercising your lungs would make a difference but it sure does.