Welcoming you
Monarchmom 
Oh so easy for others on the sidelines to say things like your son has...
"with the right discipline she'd be different." 
Try hard not to let comments like that undermine you...keep you awake at nights. Of course...that is easier said that done, too, isn't it?
A favourite mantra of mine is...
"I did the best I could and when I knew better I did better and will continue to do so." What more could be asked of a person...especially a Mom? I had to repeat and repeat that in my head before it started to grab hold. Oh believe me...I still have my days when the feeling of guilt starts to creep it. My daughter can be unrelenting in her attacks on me and it takes much fortitude to ward off those blows and keep my confidence in knowing I have been a loving Mother...doing her best.
Could we all have done things differently? Probably. Would it have made a difference as we tried to deal with the BPD-type-behaviours of our troubled children? Maybe...maybe! As much as we try to guide them towards better choices, our children, be they troubled or not, have a free will and they alone make the decisions that affect their lives. We should not shoulder responsibilities of their actions, robbing them of getting the ability to do that for themselves.
Great that you are soon to see a professional in an attempt to get a diagnosis on your daughter. Many of us here (me included) are left assuming this is the problem with our loved-ones because the check marks are all there.
My advice to you is to make sure as you go forward that you and your husband look after yourselves. That is not to say you cut your daughter loose but you make sure she does not use up all the "family oxygen." Some on this forum share that they, themselves, go to counselling, support groups, whatever.
You write that you are "ready to learn." What a goldmine you have hit upon here! As one other member so eloquently put it, this will be a marathon... not a sprint. All the work is yours to do but much to learned in reading the posts of others and, in turn, others will learn from you. Hope you keep sharing.
Hang on, Monarchmom! Here is to better days for you...for all of us!
Huat