I've been using positive conditioning for this and have had much less flashbacks the past 2 months. Maybe my method can work for you.
This process involves a dog clicker and a treat:
1. You sense a flashback is coming / it is occurring / it has just passed
2. Perform a "good behaviour action"
3. Right after performing it, press the clicker
4. Give yourself a treat
For step 2, a "good behaviour action" can be anything that you deem healthy/good for you that is low effort. For example: drinking a glass of water, washing your hands, breathing deeply 5 times, etc. Then, when you feel more capable, you can increase it to something that takes a bit longer that you'd feel more proud for doing. In my case that could be: reading 2 pages of a book, cleaning our dining table, dusting off a shelf, etc.
Performing an action that you deem is a good behaviour, whatever it is, will first work as a distraction and then, once you complete it, you'll hear the sound of the clicker and get a tasty reward. This will slowly replace the bad experience of the flashback with the good experience of performing an action you deem good and useful and be rewarded for that.
If you do it enough, the sound of the clicker itself will start being its own reward and help activate a sort of good memory loop of other times you did these good behaviours. That's how it is for me now. I use it even when I don't have flashbacks, when I do something good.
I recommend getting a clicker that wraps around your finger, that way you can keep it there without interfering with typing on keyboard and other activities.
I hope this helps.
