My girlfriend is jealous of my female best friend and doesn't want me to tell her things about her.?

My girlfriend, Anna is very jealous of her and we quite frequently get into arguments over it. She makes petty comments about Sara (my best friend) like you love her more than me or is your favorite animal the giraffe because Sara's favorite animal is the giraffee? My best friend is the one I go to confide in and I don't have many friends to talk to. Lately my girlfriend asked if I tell Sara the things she tells me. Meaning if I tell Sara that Anna is jealous and the things she is jealous about. I do tell Sara these things because I need someone to confide in and the things Anna tells me upsets me. Anna got mad and was very upset that I tell Sara that she is jealous of her and why she is jealous. She asks me why I have to tell her those things and why can't I find someone else to confide in that is like an outside party. I don't have other friends to confide in like I can with Sara. I just don't understand why Anna can't see that I need to vent and Sara is my venting source. Am I wrong for telling Sara my problems with Anna and Anna's problems with Sara. Should I find an outside source. I love them both and I want them to get along.