What I personally tend to have problems with is not rumination per se (thinking about past things too much), but that my brain/mind sometimes goes on overdrive in all directions. It's like a multidimensional hyperspace with thoughts crossing over all over the map. Past memories, future anxieties, and everything in between, including some really bizarre thoughts that I have no idea where they come from at times. As though all the filters were gone from my mind... When I'm like this, usually it lasts 2-3 days and then it passes. I do believe that it has some sort of chemical cause, but can definitely be further triggered by cognitive and emotional overload, and stress. I think all the suggestions here are good. I would add physical exercise. I am also a fan of meditation but there are states of these where it just does not work. I find that talking about the stuff that goes on in my head helps a lot - when I was younger, I used to be afraid that I would scare people away... but I tried first, then second time, and realized it's actually the opposite... there are always lots of people attracted to craziness
I also try to get me not to worry about these states too much if possible. But sometimes it's just not enough. I've visited doctors being in these states several times in my life and was told all sorts of things from generalized anxiety to panic attacks to bipolarish episodes, to just being a human being with an active mind... oh, even sleep disorders when this happens at night in half asleep states...I was evaluated for those twice and they found nothing unusual. I just decided that I will try not to worry about this much anymore and won't try to put a name on it. Just call it being me, "haennie unspecified"...
My suggestion is to experiment with whatever method seems interesting and helpful to calm your mind, just like for recovery. Except alcohol and other self-medication drugs.