Listening to an Adam Grant podcast (sorry, forgot which one) he mentioned the distinction between worry and ruminating.
In his explanation worry was thinking about things that might happen and mitigating / planning for them. Ruminating was chewing the cud but planning no action. His suggestion was a half hour of rumination max per day.
This article gives a slightly different definition (ruminating backwards looking, worry forward looking) but has some useful tips if you are a worrier.