Hello
Mom57 
and welcome
I, too, am a Mom who has a troubled/troublesome daughter.. While mine (who is now 54) has never been officially diagnosed as having BPD, the signs are all there. What about your daughter? What is her history? Has she had a diagnosis? Has she (or you) ever had counselling?
What jumped at me right away as I read your post were the words...
"The guilt is the worst." What is it that makes you feel guilty? Often we, as parents, take on the blame of our children's actions, robbing them of the opportunity to own up to what they may have done and then, hopefully, learn from them.
A lot of the work done by us here is on ourselves. The guilt you write about is a barrier that hinders any kind of positive change. If you feel guilt, it shows on you. No doubt it is something your daughter already sees...she works that to her favour...and things continue to get worse.
Setting boundaries is indeed difficult...but not impossible...and oh so necessary! To get you started, there is so much information on this website with links to more.
Once again, Mom57, welcome. Hope this first post of yours is the beginning of a new and better chapter in the life you share with your daughter. The change will have to start with you but know you are not alone as you start putting one foot in front of the other...albeit they are baby steps.
Huat