I have adhd and the rushing thoughts were only a SMALL part of it. But for me, it went much deeper than that. If you are ONLY having issues paying attention and nothing else then perhaps your sleep schedule is messed up? I know if I don't get my sleep (even on my adhd meds) im a total mess.
The again, you could just have inattentive add. I have combined adhd which includes impulsiveness etc. But of course, I'm not a doctor- just speaking from life experience I hope you get an answer. I know how hard it is.