My therapist taught me to treat memories, flashbacks as thoughts.
When you remember something or have a resentment, flashback, take a minute and treat it as a thought, when you stop yourself from going through emotions and just treat the memory as a thought, it loses its power. So thoughts come and go and you don’t have a reaction to it.
I try this every day, sometimes it works, sometimes it doesn’t. Esp dealing with malicious people who made my life miserable, I re-live the memories.