I haven't had a lot of success so far in getting a diagnosis, there's a lot of different possible things that can be wrong. Chronic Fatigue Syndrome is one place to start. I'm seeing an allergist about MCAS, I've had limited success taking antihistamines and anti-inflammatory drugs.

Mostly you talk to your doctor and read stuff and advocate for more testing to figure out why you're not able to function like before. Even if it's not "Long COVID" it definitely sounds like something is causing these problems and you should get it looked at.