Thanks for your reply!
I think I figured it out last night in talking to J. We had a few big fights this past summer where J expressed not being sure whether our relationship works for him. I think I now feel extra sensitive to feeling like our relationship is safe and secure. I think I have a fear that I will be left by both my primary and secondary partners, and so I have been emotionally guarding myself so that if that were to happen it would be less painful for me.
I am still feeling unsure what my solution is. Talking a lot with J last night was really helpful, but I am left feeling pretty raw this morning since it has become quite clear that J and B are in love. Being open to this theoretically and then having it play out in reality are different experiences for me and I am doing my best to relax and breathe and not let my heart withdraw from the people in my life that I do love and who I know love me too.