I'm sorry you're struggling so much with this. I can relate to being overwhelmed by non-stop flashbacks.
Unfortunately, the only thing I've found that works had seasonal limits. I go outside in the winter with no coat only as long as I can stand it without it getting dangerous. The whole- body shock of being cold seems to work for me. The other thing that works is a really poor coping mechanism.
I'm guessing if you want to try the cold thing and it's not actually cold outside, maybe a cold shower?
Breathing is really triggering for me, so I stay away from it. Same with meditation.
I hope you are able to find something that works for you. It can be really horrible to be mired in flashbacks for so long.