I know it feels patronizing, and I'm sure t did not intend it that way. There's actually studies out there on that particular technique for helping with nightmares. It's not immediate, and it takes work, but I've heard and read that it does work over time. I can't remember the name of the technique, but the one I'm thinking of its a three step things that is supposed to help lessen the nightmares.
It doesn't change the past. It doesn't fix what's wrong, but it helps the nightmares according to the write ups. There's a similar art therapy technique that has you drawing representations of the emotions and the situations, then you change parts of the drawing to represent how you would have wanted things to be different...
when I get to my computer, I will look up the articles I'm thinking of.

The thing with most coping skills is that they do sound stupid most of the time, especially when what we are trying to ground from its so overwhelming. I have trouble with a lot of the grounding techniques because some are triggering, others just don't bring me back... it takes time and trial and error to find ones that work.
there's something to the concept of rewriting your story to change the ending, even if what you imagine just gives you a sense of power in the moment. No, it doesn't change the past, but it can help change how you feel in the moment... it doesn't work for everyone though, so keep looking if this doesn't feel "right" (and is not meant to be a magic cure. It's just supposed to help a bit over time)...