Hi Tony
The good news is that you are aware that you are ruminating as this often can become an almost unconscious process and you seem to have good idea what the issue is.

In some ways Rumination can be seen as 'Failed Problem Solving' in the sense that the brain is trying to come up with a solution to an identified concern but cannot reach a conclusion...so you get the hamster wheel effect.

A good suggestion you may find useful is to write down exactly what the issue/problem is as specifically as possible and then the next step is to write down some possible solutions and finally and importantly take action(for some issues this may be to do nothing or it may be to speak to someone) but whatever it is then Take the Action.

Just doing this process alone can help a lot because you have created some mental distance. If after doing this process you find yourself thinking about an aspect of the problem again then remind yourself of the action you have taken even if you have to say to yourself 'Cannot be problem solved'.

It is also good to schedule and engage in activities - routine, necessary and pleasurable into your day as this is also helpful in breaking up the rumination.

Hope this helps - All the best.