I have found that flashbacks are the most annoying thing that is happening right now. Perfectly innocent remarks can trigger a flashback sometimes. Things that I thought would, don't. Go figure.
When I first had flashbacks, I didn't know what they were, nor that they were normal for what I was dealing with. When I was told that they were normal, I still had a hard time believing it. How can reliving my past in my head at the most inopportune moments be normal.
I learned to cope with the first set of flashbacks, by teaching myself that not only were they normal, but that they couldn't hurt me. Also, the more that I dealt with the issues and the emotions associated with them, the less flashbacks I had. Until they were nearly gone.
My first set of flashbacks were of abuse that I suffered as an adult. My second set were of abuse that I suffered as a child. I suppose there is a difference in the cognitive level or something, but the second set were much worse than the first had ever been. Instead of just experiencing them in my head, or like unpleasant day dreams, I was experiencing physical pain, and vivid emotional responses.
When I managed to put together enough of what I was remembering to deal with the trauma, fear and pain, I was able to deal with the flashbacks as I had the first set. It was much harder, but doable. I am still having flashbacks, but not quite as bad as they had been.