This is the same for me except I'm 36. I was very active when I got covid July of 2020 and have been disabled ever since. Nothing showed up in ANY blood tests and doctors have basically written me off. However, recently there have been new insights to extreme fatigue and post-exertion malaise. They're finding that the resting lactic acid levels of some longcovid patients is very high. You should consider getting that checked, because if it is there's actually a very easy treatment for it. And if you have it but don't treat it, it looks like a self-perpetuating cycle. And if your lactic acid is high, you should get your ammonia levels checked.

I have a home lactic acid monitor made by Edge, and it's helping me see what activities are causing my lactic acid to spike and helping me manage what I can do in a day. It's been huge. I went from being in nearly constant pain and barely able to move around my house to actually being able to go out for a small birthday lunch at a restaurant with my parents and my husband. I haven't done that in three years!
I hope this helps. I've been telling everyone with longcovid about this because for some reason a lot of doctors haven't thought to check this. It's really ridiculous.