* Stacking-sorting games. It was originally discussed with Tetris, but there are many other games in the broad category of "look at symbols and put them in their proper places." More physical activities with a similar theme (i.e. sorting blocks or buttons, card-matching games) may also work.
* Repetitive activity. Give your brain something more productive, or at least less damaging, to do over and over. Many people find repetitive crafts such as knitting help. Some find other forms of stimming more useful, and there are many options including fibercrafts such as knitting or crocheting.
* Occupy your mind in other ways. Activities which tie up your mental processes can break the loop because there's no bandwidth left for it to run in. Reading and writing are good for this. If your brain is focused on plotting, it can't also be obsessing over the crap that happened or what-iffing awful scenarios for tomorrow. People who are better at math than words may prefer sudoku or other number puzzles instead.
* Emotional expression. Sometimes you just need to go "AAAAA that sucked that sucked!!!!" in some form until your emotional center gets tired of that feeling and is finished with it. Find somewhere safe to yell, punch a pillow, throw paint at a wall, play death metal music, rip weeds out of the ground, or whatever makes you feel like you're letting it out. Venting to a good listener helps some people too.
* Sit with the feeling. Emotions are information and can be enlightening. Perhaps your brain is trying to tell you something important with all this repetition. Set a time to feel what you feel and listen for inner messages. Then switch to a different activity. If your brain keeps bugging you, try telling it, "Yes, we talked about that, but nothing new has come up so I'm busy with something else now."
* Contextualizing. A journal or timeline can help put things in perspective as part of your ongoing life narrative. For larger issues, a trauma scrapbook may prove helpful -- but smaller incidents might be collected in a binder of "challenges survived."
* Logic. Distorted thought patterns can drive you nuts. Challenge them and replace them with better ideas.
* Problem-solving. Analyzing what went wrong and what could be done to clean up the mess or prevent future recurrences can help the mind process what happened as a "finished" item rather than something that needs continued attention.
* Flanking maneuvers. Some problems cannot be solved directly with available resources, but there is often something that can be done to combat the root cause in some way. You can't fix a poor neighborhood by yourself, but you can work on local efforts to lift people out of poverty. You can't remove conflict from human interactions, but you can teach conflict-resolution skills and gather emotional first aid resources.
* Remember to breathe. Brooding and anxiety can lead to unproductive respiration that makes you feel worse. There are many breathing exercises which promote calm, relaxation, and mindfulness. If you frequently have looping problems or other brain weasels, then consider adding a meditation practice to your life, because training your brain not to bother you takes time and practice.