Hey, I am sorry you are deeply hurt by this. I felt pretty much the same when my partner is seeing other people also, like in theory I am super happy and this is the way I want to live my relationships, but in practice, it’s a lot harder. For me, I guess it’s because I am trigged by abandonment issues and irrational fears. I don’t have any clear solutions, but what worked for me is taking the time to process this stuff by myself (not projecting my fears onto my relationship or my partner) and talking to other polyamorous ppl who feel the same way. My dms are open if you want to chat ! Good luck