That's very similar to my experience. I was diagnosed with POTS and improved management of it until I got COVID in 2022 and I developed new symptoms, and my original symptoms got worse and never returned to baseline. I didn't think they'd take me because I had POTS before COVID infection, but they said since I had new and worsening symptoms, it would still be okay as they have other people in the study who also had underlying conditions that got exacerbated and they want a "diverse pool" of participants.

There is a website: https://trials.recovercovid.org/

But they don't list the participating locations. There are sites across the country. You can search for clinical trials on the NIH website: https://clinicaltrials.gov/

If you're near a research institution, it may be more direct to go to the medical center's website and see what trials they're doing and if any are for Long COVID. I got recruited through the academic medical center where I'm a patient.