Why am I jealous of my bf's female best friend?

My boyfriend and I haven’t been together for very long, just a couple months. I get really jealous of my boyfriend's female best friend. I’m jealous because they are really close and have been friends for years and do random things that you would only do with someone you feel super comfortable with. I just hate when he goes over to her house and is there for hours & they even drink sometimes, which makes me think negatively & just gets my mind going. Before we started dating & were just talking he spent a night at her house drunk & said he slept on the floor, it would really upset me more if he did that now. I don’t think anything happened, but still. He seems to really enjoy spending time with her maybe even alone. I always feel jealous about her and I hate hearing about him hanging out with her, makes me want to just stop talking to him. I dislike it when he gets texts from her & I hate how personal he is with her. I mean I have male friends but I never hang out with them like that and they're all either taken, or not my type, I don't find them at all attractive. But I’m pretty sure my boyfriend has had a crush on her (they cuddled once a long time ago) & she’s attractive. I believe him when he says they're only friends and that she’s a good person & friend, but how’s it OK to do date like things together & doesn’t he think it would make me feel uncomfortable. Maybe I'm just angry right now because I’m thinking the worst, but I feel very hurt every time he talks about her & of course when I talk to him about it I feel like a jealous ***** & he makes me feel really guilty about it like I’m overreacting. How can I not be jealous or make him understand where I'm coming from because I feel awful being like this. I even cry. Trust issues much, I really do want to trust blindly.
Why am I jealous of my bf's female best friend?
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