Have you ever tried Wim Hof breathing techniques? I also have ADHD and I have a really hard time with meditation but it works for me.
It also sounds like it could be possible that this is at least partially an issue with your body chemistry. There are so so many little things that can affect the brain, like maybe a medication or any other drugs you might take, a food you're not fully aware that you're a bit allergic to, inflammation, a malfunctioning brain chemistry such as an anxiety disorder, etc.
Here's a link to the Wim Hof video if you're interested. Just be aware that people get weirdly cultish about him, and I'm not sure I would follow everything he recommends, but this particular exercise helps my anxiety a lot.
Fun fact: When I briefly worked in a funeral home, I learned that dead people just chilling on a table will sometimes look like they're breathing. I'm not talking about actual movement, like fluids settling in the body or anything like that. It's a sort of optical illusion. I guess the brain is used to seeing people breathing so it fills in the blanks. I experienced this illusion myself and so did others working there.