Counter: mental ruminations

Imagine the following situation: you've just come out of a meeting and a clumsy little phrase you've said is going round and round in your head. It obsesses you to the point where you can't concentrate or sleep. The good news is that it happens to almost everyone. But does dwelling on a problem help us find a solution? Or, on the contrary, does it make us more anxious and tired? Why do we do it?