Hey there Emz, I've always called this "Runaway thoughts" or "Runaway scenarios" and had them for years. They still try to sneak in, but the skill I practiced and
learned to stop them in their tracks is this one breathing exercise where I breathe in through the nose for 8 seconds and say "In" to myself while breathing in, then
breathe out through the mouth and say "out" and then say "Pause" when there's that little pause. Saying the words takes up the space of thoughts entering. They
may still start to enter in your head, just acknowledge that thought, then go back to breathing and saying "In" "Out" "Pause". It takes a very short amount of time
to get this down. Another thing I do is just immediately think of and/or write down things I'm grateful for, no matter how little it is (clean sink, bird that landed on
your windowsill, anything).
The runaway thoughts/scenarios had been my biggest problem. I do take a very, very low dose of anti-depressant and an anti-anxiety that help a lot as well, but
the breathing think and the grateful thing are my two saviors from those thoughts.
I hope this helps!