There are features do seem to overlap. And while it is not unusual for people with BPD to wind up with PTSD - due to dangerous behaviors - it isn't necessarily true that people with PTSD often have BPD. (This is like one of those crazy math questions - if blue people have X, are all people who have X necessarily blue.)
I was diagnosed a long time ago as having BPD - which I absolutely kept to myself while I was in medical school. Later, I was told no I didn't have BPD, just anxiety and depression. Then of course, when I had my near complete meltdown of my life at age 49, I find out that what I really have is PTSD. I even had a psychologist who briefly thought I might be bipolar - but she sent me to a psychiatrist for an official diagnosis.
So yes - it can be terrifically confusing. My psychiatrist that I have now took a long time to rule out BPD but he feels very strongly that I don't have it. But I have found that some of the dialectic/cognitive behavioral therapies that work for BPD work very well for some of my anxiety and phobia issues as well as reconstructing barriers that were destroyed by those who abused me.
