Rumination isn’t the same as worrying.
Worrying focuses more on the what ifs - where rumination is fixated on what has already been.
Rumination stems from the brain’s desire to keep you busy - and it plays on your desire to change the impossible. Your brain likes to keep busy and likes to solve problems, even if those problems are well - in the past.
Those prone to negative thinking are easy targets for ruminating thoughts - and that’s what makes rumination so difficult to deal with.