Wow forward, that would drive me nuts as well. Fortunately nobody I know even knows about ptsd and are pretty uneducated. I tend not to get into arguements with people but my life isn't considered complicated right now.
Keeping your cool as much as you can will soon make it obvious that its their problem. Try not to give into their 'digs'. I know that the ptsd gives us a disadvantage, but they don't have to know it.
They want to focus on your ptsd, turn it around and ask them about their health problems.
They say your avoiding, tell them they are focusing on your problems and avoiding their own.
Tell them they are wrong, tell them your therapist says you are doing well.
Tell them they really take the cake, being even more messed up than you are.
Tell them you are actually facing your issues, but they just do more and more of the same.
After you feel that you have made your point and done it with confidence, tell them that you are not interested in talking anymore.