I probably have the same overawareness and too much thinking that you described. I'm not ADHD (my little brother is though), but diagnosed AS and generalized anxiety disorder. My mind is always going 100 miles per hour. I've had trouble in the past with chronic headaches, and we think it may be because I just think myself into them. Anyways...
I take prozac daily, and it helps me calm down my thinking a bit. I really like it because it doesn't have side affects (for me) or bother me in any way, but it does make me feel sort of apathetic at certain doses. (That can be a good thing if you're moody.) I've used valium, xanax, and other temporary stuff like that but I really don't like how they make me feel. Mostly they just make me sleepy. If I was trying to calm down, I would rather just take a promethazine/phenergan (anti-nausea drug with sedating effects). I have some ritalin, but of course that doesn't have a calming effect unless you are ADD. If you think you may be ADHD, it could be worth giving a try. I was prescribed it for fatigue...it has no impact on my thought processes, but it makes me move around a lot. When I take it I usually complete a lot of homework and do a lot of cleaning
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After a time, you may find that having is not so pleasing a thing, after all, as wanting. It is not logical, but it is often true.
--Spock