Thank you Washington for continuing to prove that we are decidedly the best state in the nation
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.
To answer your question indirectly, I think you need to learn to stand up and protect yourself. Those are skills, and they're learned skills. I'm making big assumptions here, but I'm guessing your parents never taught you good boundary setting. A quote that's gone viral recently is that “[the definitive symptom of childhood trauma is] trying to get a difficult person to be good to us in our adult lives.” You probably don't believe this internally yet, but you deserve to be treated with kindness. Difficult people do not deserve you.
I love this book in particular, and it's widely available for free in libraries. https://www.amazon.com/Set-Boundaries-Find-Peace-Reclaiming/dp/0593192095