See a different GP if you're not happy. It certainly sounds like your GP has been taking things seriously, but a second opinion may help convince you and also ensuire nothing is being overlooked.
Post-viral fatigue syndrome / CFS is certainly quite common, and may well turn out to be the best explanation. All sorts of hypotheses have been put forward to explain it, but it seems most likely that it comprises several different disorders with overlapping symptoms. Some patients may have a form of depression, in addition to their other symptoms.
There is no magic wand that will make all cases get better, but various approaches have been reported to help in subgroups of patients. In particular, resting in the early stages but then slowly increasing levels of activity is often cited as a successful strategy
gy. Some people may also find that analgesics &/or antidepressants help with accompanying muscle pain & low mood.
If it is diagnosed as PVF/CFS, remember that the vast majority of sufferers will return to completely normal health, though this may sometimes take months or even years.
Strangers keep coming over to pick up, help!