Hi. Firstly - Is your girl friend really lying? I have B.P.D. myself, and some of my past has been distressing to the point where I am unable to talk it through clearly. I remember parts, and talk in 'bits'. I know when a person experiences trauma, the mind blocks up at the time. Your girl friend may be experiencing flashbacks of some quite awful things. It is good that she is trying to communicate with you. It would perhaps help her to clarify things if she spoke to a professional. I don't believe she is lying, however, on the flip side - I have lied myself about certain things in my past, purely because I couldn't face the horrifying distress of the truth.
Stay strong, and patient ...most importantly - Take care of yourself. Beanie0 :-)
I fully agree with Beanie0; if you're girlfriend has BPD she may be experiencing difficulty remembering moments of time, especially if she was engaging in risky behavior as those experiences are often interpreted as "suspended time". As far as whether or not you can trust her, I think it depends on the severity and circumstances of the lies. The best option is most likely to sit down with her and have a serious conversation; tell her that you support her and understand her ilness (if she has BPD) may be partially at fault for the "lies", however, you need to be able to work through serious issues like this and, perhaps, work out her past. If the relationship and/or the lies are that serious you may want to consider couple therapy.