Yep yep. Super common phenomenon.
The technical terms are Intrusive Thoughts and Rumination, and/or ruminating on intrusive thoughts.
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Sometimes/Oftentimes these also fall under ‘maladaptive daydreaming’ umbrella. Not all intrusive thoughts or ruminations do, but fictitious “what IF” nightmare scenarios would do. The same way the opposite unicorns and rainbows “what IF” trap people often fall into would do. <<< Maladaptive daydreaming is nearly always a self-defense mechanism run amok. Whether it’s warm fuzzies or be prepared! The dissociative aspect of daydreaming is what makes it feel so real, just like a dream can feel real or one can experience real emotions for fictitious characters in a well written book/movie.
Like most things Dysreg? (Anxiety running hot spiking into intrusive thoughts, into dissociation, into panic/terror, into this/that/the other?) Your thoughts/feelings/actions lashing out randomly or in massive overreaction to whatever? The best “trick” I know of is learning to insert a pause.
There are a zillion tips/tricks/ways to deal with the individual aspects that are spinning wildly out of control… but it’s that blink of a -pause- that lets a person start taking control back & beginning to self regulate, again.
ETA… One of the cool things about PTSD is that a person CAN learn to self regulate, again. First by effort, and then it “simply” <cough> happening on it’s own. (Simply in quotes because retraining/resetting the automatic regulation? Both No-small-feat & one that often needs revisiting under times of stress) There are disorders & conditions where dysregulated thoughts/feelings either cannot be reset/retrained, but will need ongoing attention; or cannot be managed at all (without medication). IF you’re comorbid with any of those disorders? (OCD, Bipolar, ADHD, certain kinds of HFA, etc.) then learning to manage intrusive thoughts and rumination will most likely be a 2 pronged affair; using both PTSD skills/tools & skills/tools for the other disorder.