Hello jlah: I see this is your first post here on PC. Welcome to Psych Central.
Yes I think I can relate to what you're experiencing. I'm 71. I've never been diagnosed as having BPD. In fact I've never actually been offered anything in the way of a diagnosis although I kicked around the mental health system where I live for around 20 years or so (beginning around age 50.) I was on a variety of antidepressants for periods of time over the years along with a few other psychiatric med's. I've now simply left the mental health system behind & get along as best I can.
I once read the book
Lost in the Mirror. And based on what I read in that book, I thought it might be possible I could have been diagnosed as having BPD when I was younger... probably not anymore. I've been told that BPD tends to burn itself out as a person ages. And I've certainly done that. (Perhaps not in your case though?) I've also read a couple of articles, here on PC, on the subject of Bipolar Disorder II. And I can see where some of that may apply as well. Apparently BPD & BP II are sometimes confused. I've been hospitalized twice over the years. And the psychiatrist I saw during my second hospitalization said she thought there might be a "bipolar element" to what was going on with me (whatever that means.)
You wrote at around age 50 you were put on Zoloft & that somewhat contained your BPD symptoms. But before that you were a hurt angry person. I'm still pretty-much a hurt angry person... mostly at myself though since, in my life, I've left a sad trail of destruction behind me over the years. I tried seeing a few different therapists for brief periods. I don't see anyone anymore. I just keep it all to myself. I'm not "starting over" as you mentioned. I'm simply waiting for nature to take it's course... & replying to posts here on PC in the meantime.
Anyway... that's me. I hope you find PC to be of benefit.