Having been confronted with this question several times, I’ve gone into this and realized that nightmares are not something you have to live with, good counselling with can usually reduce them significantly in a fairly short time, and also after PTSD, best using various versions of Image Rehearsal Therapy, IRT.
I’ve often tried this out myself while drowsing, successfully.
(To just repeat / confront the nightmares at daytime works, but is irksome.
Lucid dreaming is also learnable and helpful, I can’t though.)
IRT means you usually start with a dream diary for two weeks (written or recorded). At daytime you repeat the nightmare, but soon start to positively remodel it.
You need some imagination, these are things I’ve read, tried out and recommended:
Sharks can be turned into dolphins,
explosions into confetti or a bursting balloon.
You’ve run someone over: It was just a bag on the street.
Someone chasing you is doing it to give you back your purse/wallet.
Find your own examples and practice while drowsing.
My personal favourites:
I’m falling and turn it into deliberate flying. Never used to fly in dreams, now I can. Helped me overcome my fear of heights, too.
Something falls over -> turn it back like a video, cf. https://youtu.be/4nj0vEO4Q6s?t=326
Challenging situations: ride them like a mustang.
Apart from that:
Treat yourself to something, e.g. drink a warm tea (thermos flask).
Reality check: Put light on, get up a bit, do a bit of housework, cool down. If applicable arrange a code-word with your sleeping-partner.
Call a helpful being: lion, horse, angel etc.
Grounding exercises: mindfulness exercise; Count backwards from 1000 minus 7 (993, 986…), Comforting: Imagine your favourite colour, music, animal.
Therapies usually only take 4 sessions und are paid by all health insurances in Germany.
If a nightmare always centres around the same topic, then a trauma could be the cause, so this should be taken care of first.
These techniques can possibly also be used for negative notions and obsessions, at least I try this.