Do you have any interest in pursuing other relationships? Just curious, because a little NRE of your own might help ease your feelings about her boyfriend.
I am in an equilateral triad, where we are all involved sexually and romantically. However, I still - to a limited extent - understand some of what you're describing. I have struggled some where sex is concerned (and have posted about that at length), because she is still new to him and we've been together for ten years. I sometimes feel like I'm "old" (and thanks to a high sex drive, also always available) and therefore less desirable.
Like you, I know my relationship with him is rock solid. What we know and what we feel, however, are not always the same thing.
So, here's what I've been doing. When my partners have alone time, I've been finding uses for that ME time. I do things they don't generally like to do. I find ways to occupy my time. I am still struggling. I still feel a big sense of anxiety when they have alone time. I have some serious abandonment issues. Everyone I've ever loved (or thought I did) has left me in some way, except for my husband (and so far, our GF). So, I think, despite knowing I'm secure with him, I'm getting some of that stirred up by the newness of our relationship with her.
All I can say is, use this forum to vent when you need to, and talk to your girl as openly as possible. I recently told them both, look, I'm still struggling with anxiety when you two have alone time. Right now, I *need* to know if they've had sex. I have NO idea why, but NOT knowing is worse than anything - I think because I am utterly out of control over what they do with their time (and should be, because it's not about me). I hope to eventually get to a point where I don't care and don't ask (or ask just because it's kind of hot to hear about it, lol).
I told them that this is NOT about anything they're doing wrong, or anything I think they will do wrong. I don't think they're talking about me (at least not often or in a bad way), and I don't think they like sex with each other more than with me... I worried about these things at first, but now I don't. But I still feel anxious. So, some nights when they finish alone time, I need a few minutes to compose myself before I can deal with being in the same room with them - not because I'm angry or hurt by them, but just because I need that time to process.
I am about as in touch with and as honest with my feelings as I can be... some things they don't need to hear (like when I'm being really crazy and thinking the worst). That's when I come here to work through my feelings and sort them out so that I understand them better and can better present them to my loves if I need to - and sometimes I do, but other times, I really don't. I just need to think out loud.