It's wonderful that you're feeling so much more solid about the whole thing, SimpleSimian!
One thing I've noticed though my experiences with polyamory is that my feelings will oscillate very rapidly and very severely. I remember one night after my girlfriend was with her lover, we were talking afterward, and I was happy and excited to hear how it went, and literally 2 minutes later I was crying so hard feeling sorry for myself, and in another 2 minutes I was back to being excited and curious!
If you're experiencing this at all just try to be strong and maintain a perspective on the matter. I find it very helpful to keep a little journal in which I write about my feelings and what I think made me feel that way. When I'm feeling unstable I go back and read through it to understand my ups and downs a little better.
I hope things go well for you tonight! We're here for support if you need us!