Hi devastatedwife,
I understand this issue very well. You're still so deep in the dysfunction of your relationship that you aren't sure whether you really are guilty of the things you've been blamed for. My relationship wasn't nearly as long, and no children were involved, so I can't totally imagine, but I remember feeling the things you wrote about: the guilt about the hurt you might have caused, the devastation at the thought that you lost him because you weren't there for him, the intrusive thoughts about your husband and his new lover, and even the confusion about whether it's over or not.
I don't have an answer, except to suggest that you think about 1. the reasons he was diagnosed with bipolar disorder; 2. the reasons you and your therapists think he might have BPD. Those didn't come out of nowhere. No, it's not all your fault, and I'm guessing that's true by a long shot. It's never all one person's fault. You might indeed have become emotionally distant. Can you reconstruct how that happened? Was it perhaps a way of protecting yourself? Is it possible that his perceptions of being abandoned are magnified by his own issues?
Think about how much of your devastation is fear: fear of being alone, of never finding someone to be with. Those feelings of fear are about you, and I believe you will recover from this if you can look inward.
I have found this workshop incredibly helpful--about 10 beliefs that can keep you stuck and in pain after a breakup:
https://bpdfamily.com/message_board/index.php?topic=136462.msg1331264#msg1331264Please take care.
-steelwork