<human>: Condense the text into a few key points

Like almost all topics see here, I feel like I'm dealing with a very personal, specific issue and but hoping there may be some broad advice from other's experience. I'm with a loving partner who I trust. Someone who has accommodated my anxiety on this issue, in spite of my abrupt 180. 

While I suffer from near constant inconvenient-to-paralyzing social anxiety, I've never been jealous or uncomfortably insecure about my partners. I've navigated a decent number of monogamish and experimental relationships (casual, committed, flings and cohabitation) in the past. Up until now these situations have been outside of my general anxiety: I'm normally confident in my sexual ability and the validation from physical affection tends to trump my insecurity. 

With my current partner, I've slept with someone else and we've had a third person in bed. However, when I was told over the phone about her sleeping with someone else (the first instance with her where I wasn't involved), I literally dropped to the ground and greyed out. Even though it was something we had discussed and agreed on, I had the strongest visceral panic reaction I've ever experienced.
<bot>: In spite of all prior experience, suddenly unable to handle my GF with someone else. Is a non-monogamist life just something I should give up on? Does anyone have any experience working through a similar situation?