It's both.... she's feeling insecure because she feels like I've been trying to create a life that's separate from her, and she feels like i'm more excited about him than i am about her (like i said before, LOTS of NRE going on between he and i...), etc. She's scared right now that I'm going to leave or, conversely, that I'm going to try to force her to accept him as another primary partner when she's not even entirely sure that being out as poly is an okay thing.
I'm totally willing now to go back and take the time i didn't take before. I know that I put us in this place where she's scared and hurt and jealous, etc., and it's up to me to make sure that she knows that I still love her and don't take her for granted or anything like that. It's a tough situation though, because like i said, i never really learned to talk about my feelings and my needs, and I'm getting a crash course in it now.
Thanks for the replies everybody, I've been reading them as they come in and I appreciate everyone's point of view.