My girlfriend is always keeping secrets from me...what should i do?

we are both15 and we've been dating for over two years (i know, insanely long time). She's had this really messed up family life, i really dont know the story, but her family doesnt love her and all these effects have sprung up from them like insecurity and that she cant make decisions easily because she thinks nobody cares. she still (after TWO years) wont tell me these secrets about her family and young life that CAUSE these insecurities and side effects, and its really been getting to me lately. we've had small fights over the issue and she says that she trusts me, but the only reason she wont tell me is because i tell one of my really close friends (goes to a different school, doesnt start gossip, really nice person) all this stuff to see what i should do, and to get feedback and to talk it out, you know? anyway, she wont tell me anything as long as i tell my friend, even though i cant see any reason why it should matter! the friend isnt telling! anyway, i can't help her work through her problems unless she tells me and she refuses to tell me...suggestions?