I have this problem where I get thinking about something, and I can't stop. My mind just can't set it down and move on. Although I haven't been diagnosed with ADHD, this feature of my brain is probably some of the strongest evidence I have that something's not quite right in my head. Self-diagnostics aside, I needed a solution, and the best solution I've found to date is to force myself to pull out something I can take notes with and write my thoughts out in detail.
I'm no psychologist, but my own experience has demonstrated to me that writing down my thoughts tends to produce a feeling of closure, which greatly reduces the need I feel to continue rehearsing the thoughts in my head over and over, making it much easier to move on to other topics.
It's also nice to have a record of my thoughts, so that I can refer back to them as needed.