One of my best friends is really mad at me!!! Help!?

So my friend, Marietta, is really mad at me right now. She keeps telling me stuff like "You are so rude to me and you always hang out with Naomi." I do hang out with Naomi a lot, but I still talk to Mari!! Another thing that I told her is that she never tries to "put herself right in front of me" when I talk to Naomi "too much." I try to hang out with Mari more when she says it, but it's never enough. And I know her well enough to knw that she is probably always mad at me because if I say something she acts like everything's okay but she keeps bringing it up. I;m really frustrated and stressed!! What do I do!! What should WE do? I love her to death (as a friend) but I'm wondering if she'll ever be satisfied!!