So I had covid earlier this year and it reactivated EBV. So a double whammy. It took me from early Jun when I was sick to November until i really started to feel good again. Like you i felt like I would never feel normal again.
My body couldn’t handle more than 30 minutes at a recovery pace on the bike. So I just took it really really slow for months, the only exercise I did was 30 minutes of riding at recovery pace or walking. I then added on a bit of body movement stuff. In October I was able to MTB again very slowly, but man did i feel to get out again. Finally, in November I feel like myself again and I’m starting to interval train again.
Just know that there is light at the end of the tunnel even if it doesn’t feel like it now.