Well, I think I've figured out what's been going on... there were a couple of things, and I feel so much better about it just realizing why I'd been so disinterested in sex.
The first issue was that I'd felt like I lost my sexual identity - I don't consider myself heterosexual anymore, and that left me to wonder what am I
? and I've been feeling kind of destabilized. Yesterday I spent quite a bit of time reading up on sexuality (I found an interesting book regarding sexual fluidity which seems to occur with some women, that has helped me feel less confused), I now feel okay with letting go of that label I had used to define myself, and I don't feel a need to replace it with some new label. I am who I am!
The other issue has been a lack of confidence in the sex-with-a-girl department. Generally I am the dominant one in the bedroom, but without having any sexual experience with women I started feeling less certain of myself once the NRE died down a little. Which I hadn't really talked to her about (it's hard enough for me to admit to MYSELF that I'm feeling insecure), so over time it led to my sexual self feeling self-conscious, withdrawn, disconnected... which feelings don't lead to wanting sex. Duh.
Once I figured that all out, I felt SO much better. This I was able to talk to her about, and this is something we will work on. I just need to learn her, get more experience, and cut myself some slack when I don't do things exactly right. I am something of a perfectionist, so I tend to be harder on myself than I should be.
Anyway, thanks for the talk! Sometimes I just need to get things out of my head, and stop concentrating on the trees so I can see the forest.