My best friend ditched me for her ex-best friend?

Ok so let call my bestfriend A. A had a major fight with B-her best friend at that time. When A was down and upset I was always there to hang out with her and encourage her as well as giving her advices. And we were really tight, she even invited me to her exclusive birthday party in universal studio during summer. And then 2 days ago, A and B kissed and made up. They became twin-like again and it's so hard to come in between them, I do feel that I don't belong there in between their convos.. I love A, but now I suddenly turned into a second choice, an option. I feel like a sucker, am I supposed to hold on to A? I love A with all my heart and because all my old friends turned against me so I don't know what to do.. Maybe this is so important to me because I like A and I hardly ever like anyone..