Hi 55suns,
It's tough to be on the receiving end of those opposing emotions. A person with BPD tends to think in black and white/good and bad – there isn't much room for the complexity of human qualities in one person.
Also, it is quite common for a pwBPD to project his/her own negative feelings onto a partner or someone else close, which it sounds like she was doing when she described how bad she felt about herself. It has to be someone else's fault, because experiencing that core shame is very, very difficult. Try to remember that it is not personal, it's really a coping mechanism for her. She has to protect herself from those feelings of shame and defectiveness.
In my relationship, we could be having a great conversation, and a (neutral to me) word could trigger an extreme reaction, where he felt afraid that I was going to hurt him and he had to protect himself from me. Then, minutes later, after processing his emotions (and me supporting him as best I could) I would be "wonderful" again in his eyes.
We have a good workshop on this topic, if you'd like to check it out:
BPD BEHAVIORS: Splitting