I think I can embrace my partner having other partners. I will go even further and say that it is likely that my partner could have a partner that might provide her with something I can't. But what was messing me up was that she was spending 7 - 10 days in a row with the new guy, and I was lucky if I could get here to visit me twice in the same week - just quick visits; sleepovers stopped completely. I asked her for more of her time, and she responded by pulling even further away. It really hurt - I felt like I was mattering less and less to her.
I like to have a few days during the week, and more often than not, one day of the weekend, as it is my sacred time away from work. That isn't some kind of requirement, but just my negotiable preference. The problem is that every time I tried to have a negotiation it was taken as a criticism or attack, and communication just fell apart.
My GF told me she had some kind of learning disability that made communication really difficult for her, but when things were more casual, it never came up. It seems like while I'm reading through this forum that communication is critical. It also seems like getting past these emotional hurdles seems to work better with an effort from both parties.
Last week I truly felt that my polyamorous experiment was a failure because of my makeup - but I really cannot stress how jacked up our communication was - truly dysfunctional. Now I'm not sure if the root cause was or total inability to communicate.
I'm also hesitant to just accept that I'm doomed to feel insecurity in a relationship when my partner is seeing someone else. I have processed my way through petty emotions in the past - I'm not convinced this one is different.