This may not be your lungs at all. It could be any of the following, but I'm just letting you know that there are other possibilities...NOT suggesting I know what you have going on!!! !!!

You could have a mitral valve prolapse severe enough that you're getting severe regurgitation. Normally this is suspected and it should have been ruled out.

Or, you have a damaged or partly occluded aortic valve. My own well-aged dad was having trouble walking and kept having to lean against a utility pole on his daily walks during increasingly frequent pauses. When they did an angiogram...POW!...they found that his aortic valve was almost 90% closed up with calcium deposits. It's a wonder he could stand up out of a chair!.. It was replaced in what is called the TAVI procedure, very common, only a day patient thing unless you're older or have other risks. He had to stay over night, being a spry 93 year-old fella.

Some people develop a hole in the septum between the two atria. It is a thinning at first, then age makes it worse and eventually the septum lets go and you get cross-wash between the two upper chambers during the beating process. If there's a heavy exchange between the two atria, it means oxygen-poor, CO2-rich blood from the right atrium might be being forced into the left atrium, although usually the problem is blood flowing in the other direction.

Once again, just using some anecdotal information of my own experience and from posting from other heart patients on other fora about what was the matter with them and what was done to improve them.