I gather from the OP's remarks that he may have been involved with someone who was diagnosed, or had exhibited behaviors consistent with BPD. Most people are unaware of BPD, and assume that their SO's erratic behaviors are due to moodiness, depression, bipolar, antisocial or anger management issues. Another term for BPD is emotional dysregulation. They seem to have a longing to be with you, to cling to you, due in large part, to fear of abandonment, stemming from traumatic events suffered in early childhood. They seem to have almost empathic power to read you and push the right buttons. They can seem totally normal, or quirky in a charismatic way, in work relationships; but personal relationships end up on rollercoasters.

I was also unaware, until I ended up on an EH date with some one whom I strongly suspect of having BPD. He was initially incredibly charming, funny, and attentive, crossing emotional boundaries. But he was also impulsive, dysphoric, dishonest, and in one single phone call, moved me from extreme idealization to devaluation, and raged at me in multiple subsequent e-mails and phone calls.

I think it's important that the OP recognize what it is about him that may draw some one with BPD to him. He may have codependent issues, be attracted to some one who wants him to be her protector, her knight in shining armor, who adores him, who may be "out of his league" but treats him like her hero, who puts him on a pedestal, who wants to cling to him.

Go slow in the relationship, don't get sucked in by all the physical and emotional energy highs that could be generated. Look for signs of sudden moodiness or anger, followed by acts of contrition. If you start walking on eggshells, ask yourself if that's the kind of relationship that you want.