hi, firstly i would say my best wishes to both of you
so the (nre) is coming closer and it is making you anxious. it would make me anxious as well. have you talked to your boyfriend about this? to me communication is one of the most important things in a relationship.
to me it would bother me if i went to a kink event with one of my boyfriends and he had sexual relations( lol i can't believe i am saying it this way ) with another person. the reason that i would feel this way is that he went to that event with me. That being said in my relationship we have a rule that prior to patrick engaging in anything out of the relationship he tells me/ asks my permission. i placed this in place at the start of opening our relationship so that i would be aware of who he was sleeping with and being that i was first i could help him in his decisions. to me i didn't want to be cheated on and this was the way i got rid of those feelings.....for myself. the other reason that i requested this be one of the important factors was that i have seen it work very very well. one of my really really good friends has had a successful open relationship with this one proviso for well over 10 years. that said if this had happened i would be upset because i went to the event thinking that it was going to be a special time with my boyfriend and me and he ended up going elsewhere on our special weekend. have you truly dealt with your feelings of betrayal in regards to this. healing this rift may take more time and work.
that said, it seems to me that either your relationship is fairly new or is still in the progress of further development. it may be that you may be worried that you will loose the time that you currently get as yours. being that is sounds that you have so little time already i can understand how this will effect you. i take it that you love your man lots( i love my two), the loss of the contact maybe something that maybe causing a bit of your anxiety as well
my best suggestion to you is to sit down and discuss this with your man. maybe you need some clearer boundaries set. a open relationship is different from a poly relationship. and a open poly relationship is uber complicated. i know that there is more stress with a poly relationship but there is more beauty as well( to me anyway). something that is big to me is that there is complete openness and communication.
the other thing to me that might be a bit of a stressor is you don't know this women well enough to know who she is. you don't know what she expects from your partner or how she truely feels about you. you don't know if she will be respectful of your time of be a complete meanie head(yes i know i am not a kid).
anyway i truly wish you the best, big hugs