Old Friends

I'm quite odd now, since my old best friend is now friends with my circle of friends. It was one thing when I introduced her to them all, but now it's like I'm not even there. Now, I get to be paranoid that they like her better, are turning them against me, and that I no longer have a place. Well, I have never felt as though I had a permanent place, but when she's around I feel more likely to be the next monkey pushed off the bed.
I can't just tell my friends to no longer associate with her. They've a class with her, and get along with her. Plus, our falling out started with me telling her that it was me, or the guy she tried to set me up with, and then fell in like with. But I want to so bad. Ultimatums aren't the smartest thing, but then I'd stop doubting. But, I sincerely doubt I'd stop being so insecure.
I'm pretty sure part of my issue is my lack of a solid best friend. That's what she was, choosing me over her other friends, hanging with me all the time. Her family was pretty much the only thing she chose over me. I've never had that. I've been the best friend in school, or best friend until someone better came along. I want loyalty and trust, and I want it fully right now. I'm impatient.
TL;DR- Ex-Biffle is friends with my friends. It ties my stomach in knots.

TGU