The therapy take is that this feeling comes about because you grew up in an environment where it wasn't emotionally/physically safe for you to relax. Even as an adult with well managed/medicated ADHD, the subconscious parts of your brain are still trying to protect you, so no relaxation allowed. imo, this is extra common in ADHD folk because even parts of our own brains were mad at us for not having executive function.

In particularly bad cases, this can rise to the level of being C-PTSD. Feeling unable to relax is a big indicator that whatever is going on in your head needs to be addressed. Therapy is expensive, but there's ways to get it covered by insurance, especially if you have an ADHD diagnosis.