Tiddletat I can understand your worry about having done irreversible harm. If I can just share my DD's story, it might give you some hope. Sorry it's so long!
DD initially had PVFS and although we thought we were managing it ok, it developed into ME. For DC it's diagnosed after 3 months of unrelenting fatigue, 6 months for adults. On reflection, she probably had it earlier and by the time she was too ill to go to school, it was already ME.
We immediately cut back on everything and prioritised rest and rest. She was housebound and pretty much bed bound as the level of activity she could sustain without payback was extremely low. Over the years though, the level of activity she can manage has very slowly increased. We would periodically try introducing something new to see if she could sustain it. Sometimes she could, sometimes it was too soon. But it's my belief that it's the recovery (ie. time) that leads to increased activity capability, not the other way round. Hope that makes sense.
After 2 years she was well enough to take an exercise test called a 2-day CPET. The test deliberately invokes the physiological response to exertion and tests it again the next day to measure its lasting effect. Day 1 showed that she had the functional capacity of a 90-year old woman. Day 2 showed that her functional capacity had worsened. This is an abnormal bodily response to exertion that only exists in ME. It's what is known as Post Exertional Malaise (PEM). It's not just feeling worse after exertion, but your body's energy function is actually impaired and your energy envelope has literally shrunk as a result of overdoing it on day 1.
So we concluded that every time DD ran her battery down to zero or beyond, it stressed her energy system and made it worse. It showed that stressing it didn't increase capacity, but reduced it. We thought that doing this continually would be putting pressure on it and it was never going to recover. So we carried on not stressing it, not running her battery down to zero, and always staying within her energy envelope. During this time, we were able to gently increase her activities, but always on the basis that it was time and rest that had contributed to the increased function, not the other way round.
At the end of last year, she repeated the CPET. This time, day 1 showed she had the functional capacity of an 80yo man. So she was still very debilitated. But day 2 showed that her functional capacity had improved. So her energy system was no longer failing when stressed. We firmly believe that because we left the energy system to recover without continually stressing it, it spontaneously reset itself and is now working properly.
However, she still has very poor functional capacity. But we know that she can now gradually safely build her stamina and fitness level because her body now has the correct response to a bit of pushing itself, rather than the opposite. But then the lockdown came and it's all come to a crashing halt and in fact has slipped back quite a bit! 
But the main message is that leaving the energy system unbothered has seemed to help it recover and right itself. And is a good basis for why rest, rest and more rest is the key to recovery.
And if there's one other piece of advice I can give people in this situation it's to prioritise your mental health. Because the mechanisms to recover from depression are the opposite to PVFS and so it's a really difficult position to get out of once you're in it. That's hard at the moment in lockdown, but we prioritised social contact with DD's friends over school, and if it was ever a choice between a run out in the car or a shower, the change of scenery won every time.