I can understand where you are coming from - it is hard to stomach the fact that your boyfriend is emotionally intimate with another female, that he is able to confide in her rather than you but she is just a friend and he has never given you a reason to doubt him. Your jealousy is self-destructive, you can't dictate whom he is allowed to be friends with and whom he can't associate with - your behaviour will end up pushing him away. Guys don't usually feel comfortable dissecting their relationships with other guys, it's not a bloke thing, so he obviously finds it handy having a female friend whom he can turn to when in need of advice. Reassure yourself that he is fact discussing you with her, you should even be thankful that he receives opinions and inputs from a female perspective.
Instead of viewing her as a threat, befriend her. Find ways that all three of you can hang out so that you can get to know her better and who knows you might even grow fond of her. And most importantly communicate with your boyfriend in rational manner, make him aware that he doesn't always have to rely on her, that you are his girlfriend and more than capable of lending him an ear as well as a shoulder to cry on. If anybody, he should be able to discuss his fears and thoughts with you, that is the whole point of having a girlfriend.
[SIGPIC][/SIGPIC]
Love is like a merry-go-round: you get all dizzy, and then you feel sick!