My "best" friends and I are having issues. Any help?!?

okay, here's the deal. I have about 4 best friends, and two of them are giving me MAJOR issues. let's say one of them is called "Anna", and the other is "Diana". I have known "Anna" since 4th grade, but we became best friends this year. "Diana", however, just came to the school this year. We were like a trio, always together. But, now, "Anna" and I have realized that we have more in common and we are talking more now. "Diana" is getting jealous, and she thinks I am "stealing Anna away" (which is completely ridiculous), and it is making me very angry. I think this is becoming a fight between "Diana" and I, and I'm afraid I will lose. I don't want to fight, but I'm getting very irritated by the way "Diana" is thinking, and I don't know what to do, because I can't go to any of my friends, or family with this. Pleease give me some advice...