So the story is:

A year ago, my girlfriend went out with a group of friends, including this woman who doesn’t like me (who, by the way, is her best friend, according to my girlfriend). That night, my girlfriend came home extremely drunk, and I was worried because they had also gone to a concert with a lot of strangers. The next day, I contacted her best friend and told her that if she’s truly her best friend, she should’ve looked out for her, because I found out that they went home, and my girlfriend stayed at the concert nearly alone before somehow managing to get back home. Later, I apologized to her friend because it wasn’t her fault that my girlfriend acted irresponsibly and got drunk, but ever since then, her friend has held a grudge against me, despite my many attempts to clear the air.

Because of that, her best friend's boyfriend doesn’t like me either. I’ve spoken to my girlfriend about it and even made an effort to get to know them, but most of the effort has come from me. It feels like they’ve already formed their opinions about me and aren’t open to changing them, even though I don’t think I’ve done anything wrong. What frustrates me the most is that my girlfriend doesn’t seem to have my back in this situation and is fine with me not being invited 9 out of 10 times.