Hi Thunder,
I too attributed my son's behaviour to teenage rebellion, and looking back I have some of the same feelings of guilt that you expressed. The good news is that she reached out to you at her time of crisis. And, you have been good at setting limits in the past. Even though we have things to learn about BPD and may handle things differently in the future, at least we know how to set limits.
When I look back I often think that if I did things differently what really would have changed? You could have struggled to help her and struggled to get her away from the situations that she put herself in but would you have been successful or would she have been strong-willed enough to do what she did anyways.
My son too was out of the house at a young age. It seemed to us that he just didn't want to work or take care of himself and we felt that he had to learn that if you don't work you don't eat. He spent some time wandering, spent some years busking for a living and now he actually holds a full-time job. He pulled himself out of it. Still not easy, he's angry at me that he's not well off and that we lived in a nice house while he was homeless. He is now 31, and me until he was 30 to realize that he was BPD. For years I just thought, oh - he takes after his father. All the ranting and raging and anger management issues etc. Now I realize that his father shows traits of BPD as well. Now that I understand BPD and have compassion for the emotional disregulation and other issues I can relate to him much better.
This could be a good turning point for your daughter. I encourage you to read all that you can, get some good books and use the resources that this site offers.
On a side note - like your daughter I too took off with an older man when I was young. Our relationship started when I was 15 and he was 38. My parents found out when I was 18, kicked me out of the house and they were so upset they moved across the country. I eventually married the man and he is the father of my two children. We ended up having a very sucessful business together but in the end it didn't last. I don't believe that I have any of the traits of BPD and so maybe that is different but I do believe my parents did the right thing. They didn't let me ruin their lives, didn't fight with me about things. They tried talking to me, had elders of the church try talking to me and when it didn't work they "detached with love". We ended up having a good relationship in time after that. They set limits and stuck to them.
Again, I hope this is a turning point for your family. All the best!