I ruminate too. I think some people are just more prone to ruminate. I tend to do it more when I'm alone. Distraction helps me, like doing a crossword puzzle or reading a book. Physical movement also helps - even just doing the dishes. And of course, being around others. You could assign yourself a 20 minute slot each day, just to ruminate. When the time is over, go do other things. Meditation has also helps me focus my mind.