I have attention/concentration problems, and allegedly "severe" combined ADHD (hyperactive-impulsive and inattentive). I'm not so sure I have "severe" ADHD, though. I think it gets worse when I'm manic, so I can see how my attention/concentration problems can be "severe" at times. Then on top of that, I have bad anxiety and psychosis, both of which impact my attention/concentration as well. And even then, depression does it too.

In your case, it really could be anything, but it sounds like you're having racing thoughts. Anxiety can cause racing thoughts if it turns out ADHD isn't the cause of them. In fact, anxiety can cause fidgeting, restlessness, inattention, etc. just like ADHD can. So they can look pretty similar sometimes, and many therapists will tell you that. Plenty of people with anxiety are misdiagnosed as having ADHD, FWIW. So look into anxiety as well. Maybe get a psych evaluation to figure out what the root cause of these symptoms are.