Hey sbr, so glad you are here! I hear your pain at seeing your ex and his new gf today and am so sorry for that. I can only imagine how much that must suck. Again, I am glad you are here with a place to share it.
I am quite new to this whole bpdfamily so am not a reliable source of info about if BPD can manifest itself late in life. But it is very hard for me to imagine that the problems with your ex were "just you" and that he'd somehow gotten it right all the other times before (nevermind that those r/s didn't last).
It seems that many of us here question ourselves (relentlessly) wondering if it was "just us" and wondering if/wishing we could have somehow done something different to make it turn out different. But in my short experience with my UxBPDbf, it was definitely not "just me." Did I contribute to the problems? Yes. For sure. But I am certain the most perfect version of me could
not have made that r/s work. No matter how I changed or what I did.
And IF, somehow, I could have technically made it "work," it would never been easy, relaxed or
fun. Plus, I am certain the perfect version of me wouldn't have had any patience for that degree of BS of non-stop crises, drama, chaos (with a few good times mixed in (about 30%)).
So I say NO WAY it was all "just you." Not possible. Relationships take two. So, at the very minimum (and maybe more) it was at least half him!
No doubt, it hurts seeing him with the new gf. But, the way you describe it, sounds like your 18 yrs with him hurt more.
My r/s with him has been 18 years of stress and turmoil. I walked on eggshells that whole time. I cried more in those 18 years than I probably ever did as an infant. I only wanted him. I wanted it to work. It didn’t.
Remember you said the thing you have been fearing since he left (seeing him with the gf) happened today. You survived. It may suck. You may feel hurt or spun out. Understandably so. But you survived it. Are surviving it. Congratulations on that. The first time is over. You did it. Hang in there, sbr. You done good!