What rumination is and why it sticks
Rumination is repetitively going over the same worry, mistake or question without getting closer to an answer: replaying a conversation, asking "why did this happen to me", or running the same what-ifs. It feels like problem solving, which is why it is hard to stop, but it rarely produces a decision.
Psychologists Susan Nolen-Hoeksema, Blair Wisco and Sonja Lyubomirsky reviewed the research in 2008 and found that rumination is linked with the onset and persistence of depression and anxiety, worse problem solving, and lower motivation to act. Edward Watkins has shown that the style of repetitive thought matters: abstract, evaluative thinking ("why am I like this?") tends to be unhelpful, while concrete, specific thinking ("what exactly happened, and what could I do next?") is more constructive.