I can sympathize with your situation. I was a committed amateur athlete before coming down with my present illness three years ago. The most troubling symptoms have been persistent sore throat, flu-like symptoms, PEM, IBS and general exhaustion.
In spite of not feeling at all well I tried to still train during the first year of my illness, but each time I pushed a little I slipped back to a persistently lower level of functioning than before I pushed.
For the last two years I've only done light exercise and in much reduced quantities. Although I'd love to be able to do the racing my brother still does (we once shared many experiences that we no longer can), there are other less active things in life (such as my handicraft that I once earned a living from) I want to be able to do as well, and if I went out and attempted to train again I know I'd almost certainly end up so debilitated that I couldn't do any activities at all. As things are at the moment I can generally do a few hours of my craft a week and it is a lot more rewarding than being able to do nothing.
If you're an athlete with this illness, sometimes the desire to go out and train has to be slowly pummelled out of you. During periods of payback for your excessive activity you may repeatedly find yourself having to spend long periods in bed or on the sofa, and not be physically or mentally up to doing anything at all. It's then that you realise that being able to do some kind of activity at least is better than being able to do nothing, and if this means you can never train or race again then so be it.
In my youth I had a lengthy bout of what was diagnosed as PVF, but eventually recovered from it and was well enough to train and race again after about ten years. Then I fell ill again. This time round, the illness has hit me much harder, and I've vowed not to entertain the idea of training and racing again in the future, however good I may feel, until such time as the illness I'm presently suffering from is properly understood and a dependable cure is available for it. Given my age, that might be never, but in the meantime a life of restricted activity is better than a life with no activity in it at all.
Echoing what others have said, I advise you to be sparing with your energy. You're going to need reserves of it anyway just to deal with the indifference you're likely to encounter at the hands of the NHS, together with their total failure to grasp the impact this illness is having on your life.