- wear a small rubber band or something similar on your wrist. If you notice yourself ruminating, give yourself a little flick with it - the very slight pain signal pulls you into the here & now!
- designate a time and place to ruminate a bit when you feel the need, but have a timer running, and do something else afterwards. Controlled release in a way!
- generally, if you catch yourself ruminating, try to think of something indicating "STOP" or similar. Personally I imagine a big STOP road sign and also "shout" it in my internal voice.
It might sound silly, but after actively training for a few weeks you'll have much more freedom in your thoughts!