Girl's Behavior

Girls, Help! I can't get along with my girlfriend's best friend? This friend is a bad influence and they abuse substances together. Help?

I am in my first serious relationship with a lesbian and it is going absolutely great except for her best friend. Take note, three girls here. Now, respect- they have been in a friendship for much longer than we have been dating. I get that she loves her. However, my girlfriend told me once that they had a passionate night together. They have been best friends ever since. Oh, and they live in a communal home. Now, own rooms, but sometimes she would mention that they slept together. I don't necessarily think anything romantic is happening, but the thought of them cuddling, even as friends, when she's my girlfriend, and she is well, also a girl, makes me sick. And how do they say goodnight? A kiss. Eesh. Not comfortable with that either. She was happy to stop this, I don't know, while I was there? Coockoo jealous girlfriend brain overdrive kicking in. I tried to engage with a few meaning conversations with this friend and the last attempt she admitted to me that my girlfriend rarely spends time with her which made me feel guilty a little later that night. I took a good pic of her in the afternoon. I sent her a message to apologise that it might have felt like I have stolen her best friend from her. Add extra fluff to letter. However, note that my girlfriend's a bartender and this friend an online teacher late in the afternoon. They spend the whole morning together, every day. I'm a teacher so I work until 10 minutes before my girl's shift starts. Sad. So yeah. She's been sleeping over at my house a lot. Anyhoo. She hasn't replied and posted the pic I took of her on her socials. It's been months and she lives in a different country now. But the friendship is still going strong, emotionally over the phone and texting over socials. My girlfriend posted a pic of us on Vday and the next pic was of them together also a Vday dedication. And I'm scared that they discuss our relationship. I'm just overthinking. But I want to help my girl grow. This behaviour hurts me.

Girls, Help! I can't get along with my girlfriend's best friend? This friend is a bad influence and they abuse substances together. Help?
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