What do I do about my boyfriend and my friend being so close?

Okay so I'm not really the type to ask random strangers on the internet for advice but I want some opinions here.

I have a friend and a boyfriend who know each other. I introduced them when me and my boyfriend first got together and they hit it off pretty well. At first I thought it was great because we could all be together and no one would be uncomfortable. And it was like that was for a while.

But then me and him started getting into fights. I'd go to her to vent and get advice and she'd tell me that HE already went to her and explained the situation and she'd tell me how I was wrong. EVERY SINGLE TIME we got into argument this would happen, not just a couple of times where I may have been the one at fault. As soon as we get into a fight, she's the first person he goes to. Even if its 3 in the morning he'll call her.

And vice versa; I'd get into a disagreement with her and go to my boyfriend to vent. And she apparently already talked to him about how horrible I was being and what's even worse is that she'd completely lie and exaggerate about me and he'd believe her. Then he'd come to me asking why I was being so mean to her. But they were all lies :(

So either way, if a disagreement happens with either one of them, the other one automatically is on each others side and I'm left out in the cold. Its nice that they're friendly with each other but at the end of the day he's supposed to be my boyfriend and she's supposed to be my friend.

They don't ever show signs of liking each other or dating, or sneaking around behind my back. I could be wrong but thats not what I'm worried about. The only stuff they do behind my back is talk about me. Bad stuff.

I realize it was a mistake to introduce a best friend to the guy I'm dating but what am I supposed to now? I've known my friend almost my whole life (I'm 19) and I've known the guy for 2 years. I tried talking to both of them and they both deny being so close always being on each others side. They say they're always on my side but they definitely are not. When I talked to my boyfriend he said he goes to her because he has no one else to talk to because his friends don't listen like she does. I know this sounds juvenile but she's still MY friend, and she was my friend first.

I need advice on how to handle this!