@kb49
I am not a cardiologist or medical professional just can pass on my experience.

I have had a ICD/Pacemaker for over 20 years. When I first got mine I was under tremendous stress and anxiety. That caused me have a lot of fear.

Stress and anxiety can affect the way you breath. The fight or flight syndrome. Do you feel stress and anxiety when you get short of breath? Have you tried deep breathing techniques like yoga deep breathing to relax.

I mentioned these because I can remember feeling short of breath and noticed I was feeling a lot of stress and anxiety. So I worked with my doctors and psychiatrist specialist to find activities to reduce my stress and anxiety and psychiatrist put me on a medication. I do water aerobics 5 days a week and also swim and bike ride.

If you don't like an exercise activity how about a hobby. Anything to help reduce anxiety and stress is going to help. Hopefully you do not have a medical issue causing this but seems you cardiologist has ruled out pacemaker or heart.

In 2015 I got diagnosed with PTSD anxiety/panic disorder. That is when I reached out for additional help from my doctors and specialist which I mentioned above. And to this day I do deep breathing anytime I feel stress and anxiety coming on or start feeling I am getting short of breath or breathing to fast.