What can you do about people who like to bring up your past traumatic situations in casual conversations?

I just stay away from such people. They aren't really friends anyway or they would have enough compassion to leave the past in the past. If avoiding such people is not an option, the "look" of death and a quick exit works wonders on some folks However, there are some that just seem hell bent on re-living horrible experiences, so if that is the case, just make some comment to them that points out THEY are the ones that enjoy such conversations, but you can definitely live without ever mentioning it again.