Reading your story, I was constantly amazed at how similar it is to my own situation. I also have a very good friend who I used to hang out with all the time. Then another girl (Y) who we knew from college moved to the area, just a couple of miles from my friend, and now they are attached at the hip. Y is very jealous of any time that my friend and I spend together, and makes it very unpleasant whenever we all go out as a group. A few months ago, I finally just quit going out with them altogether, and when my friend asked why, I told her that Y and I are just two very different types of people, and that it's very hard for me to get a word in edgewise whenever she's around. My friend either couldn't or didn't want to understand that explanation, because she still plans girls' nights out, wanting the 3 of us to go out together and gets upset when I turn her down. However, anytime that I try to plan something just the 2 of us, she never has the time.
Finally, I just gave up. I don't call my friend and I don't e-mail her. If she wants to see me, then she has to make the effort from now on, because I'm through playing high school games. I've found that hanging out with other girlfriends is more fun and offers much less drama than hanging out with X and Y.
I suggest that you do what I have done: hang out with your other friends, meet new people, and let the two of them just entertain each other.