Hi yogabrain,
Welcome back

My SO's D19 is bipolar/BPD. It's quite a combination.
Have you read In Search of the Real Self by James Masterson? That helped me understand what was driving the pwBPD fear of being alone, why it is so acute.
Creating an environment that is more or less stable and secure can make a difference. PwBPD tend to externalize emotions, so relationship skills that help create a feeling of security, adequacy, validation -- these can improve the relationship.
If you are scared, he will be too. If you take care of yourself, he may be more inclined to care for himself.
We're here to walk with you.
LnL