@Kitcat122 although I am generally feeling much better my shortness of breath is no better. I feel like my lungs are too small.
Ok, I'm going to tentatively put this out there as my experience. I felt right from the end of my initial viral illness that there was an extremely fine line to be walked between rest and exercise. While too much exercise was a sure fire way to trigger a relapse, too much rest caused a slow but sure deterioration. While anyone experiencing CFS symptoms needs rest as an absolute priority, if your symptoms are mainly inflammatory, as mine were, the right exercise can help and be necessary every bit as much as the wrong exercise is damaging.
I made many, many, many mistakes along the way with this. But once I got a reasonable handle on what helped and what didn't, as I stepped up my exercise my breathing got better. The muscles in your diaphragm are exceptionally susceptible to disuse atrophy. Too much rest can very much make breathing harder and shallower. I found that walks were absolutely essential and I gradually picked up my distance and pace, focussing on good breathing as I did. Then I rested big time afterwards. There were days I had to take as a day of rest but if I rested for too many days, my breathing deteriorated regardless. I've also found that my upper back muscles have become incredibly tense due to months of pain from breathing. So I've been focussing on relaxing those which has also helped massively.
I'm not completely better. I have mild costochondritis and esophagitis, especially in my neck. But I'm able to manage those despite being back to high activity levels where I'm averaging at least 2-3 hours of cardio and 1 hour of strength and/or stretching every day. The more I exercise now the stronger my breathing becomes and the more I'm able to rebuild my chest muscles, the less limiting the costo pain becomes. There are still a lot of things I avoid or build up very tentatively. A lot of the stretching exercises I do aggravate the inflammation in my throat, so I build them up slowly, so my neck muscles can lengthen and strengthen and take the strain off my oesophagus. My chest and back are weak and tense, so carefully building them is taking strain off my sternum. It's a fine line to increase the strength of the muscles around the inflammation without triggering a major flare up of the inflammation itself. But if I don't exercise all the muscles that have atrophied through my illness, that atrophy is definitely responsible for many of the lingering symptoms.
It's very, very tough going through this alone with complete trial and error, and I have pushed too hard and triggered many relapses. But I once I got a handle of the balance my recovery accelerated beyond my wildest dreams. And again, anyone with CFS symptoms needs rest more than anything because exercise has the potential to cause a relapse far, far worse than any of the problems caused by atrophy. But anyone not experiencing fatigue might very well improve with the right mix of targeted exercise and rest.