Reoccurring nightmares are not you. They are someone else, like you, but a different you, in a different universe/timeline (think Jet Li in The One).

For some reason, our brains sometimes connect with these others while we sleep. Maybe it has something to do with a perfect alignment of brain waves or something, but this is also why time is often so screwy in dreams, or the facts of the dream change. You are like an old analog radio tuning in a faraway station while driving through the mountains. Sometimes the signal is strong, sometimes it is barely there, and sometimes you cross over a hill and suddenly you are listening to something completely different.

The next time you wake from a particular nightmare, stop and think about what you can do to help your other self. Make sure you take the physics of their reality into account... they are somehow falling to their deaths because they jumped too high? Then they must have the strength or low gravity needed to make that happen. So maybe it is only scary because you are applying your physics, but in their world they just need to brace for landing, or prepare to tuck and roll.... Did they get attacked by a nightmare creature? Well you just need to make sure that when you reconnect with them the next time, you reconnect earlier in the timeline, and firmly hold the idea they need to show up ready to fuck that creature up. Don't know how to fuck up a 30ft, fire breathing, strap on dildo wearing, pigeon? You don't need to. They live in that reality, I'm sure they know how to deal with it, they just need to know to be prepared. And even if they don't know, one of the other yous will. It's a pest that other other you deals with daily on the way to work. You just need to connect with one of them first, get the information, and then reconnect with the other you... odds are that the solution is a fire extinguisher, an even bigger strap on, and a bag of bird seed, and boom, you've turned their hell into a way to shorten their daily commute by a factor of ten (once they get a saddle for it of course, safety first).

Once you have your plan, you go back in with the intent to purposely reconnect to the nightmare and give the other you the tools they need fix their situation. And don't forget, time is not linear between universes. Your nightmare is reoccurring because you repeatedly keep reconnecting to this particular moment. So that also means, that with a little effort, you should be able to connect earlier in the timeline too.

Remember, it's not you, but you can help. You have all the time in the world to think it through and formulate a plan. And you can keep trying until you get it right.