Relationship Anxiety over Cheating...?

Hello, I'm actually very shy about telling others about this but I really feel like it's eating at me everyday. I'm just a young woman with so many things running through my head. My anxiety gets so high it makes me panic at work or sometimes when I'm alone. I can't let it stop me from living my life, if you can help me I'd appreciate it. You don't know how much it'll help me get over this issue.

You see, I've been dating my girlfriend for a little over a year and things have had rough patches. I have anxiety so I constantly text or message her all the time, I'm not being clingy but I just need her for support. Whenever she ignores me or doesn't reply I start overthinking things that she'll replace me. I've lost so many friends because they replaced me. We've been able to work things out and all. But, I recently found out she and her best friend engaged in something very bad together...you can say it was like a "one night stand" if that's what you'd call it. It happenend two months ago.They have had a past before her and I were together but as soon as her best friend had feelings for me she stopped doing things with her. The reason things happened that night was the result of a fight I had with her and then her best friend started asking her for sexual things. She promised me she wouldn't do anything like it but those promises didn't stop her. She felt pressured into doing things for her best friend so that she would feel better. Her best friend had a strong urge for touches as well and I won't go into detail about what had happened but they both had "girl on girl sex" and from what they tell me, they both hated it. They both had so much guilt and shame after it happened. But what scares me is how easily they were able to talk to each other almost like it never happened. And the fact that they hid it from me is what kills me...and both have been telling me different things about how it started and it's confusing me...both are guilty regardless. My girlfriend told me her friend was persistant and she didn't know what to do...so she gave herself up for her. She was very scared during it and couldn't say "no" or "stop" to her. And her friend has been telling me it started from my girlfriend because she was persistant but blames herself for it starting. So I have reason to believe her friend was the real cause...

Ever since her friend told me what happenend I started having so much anxiety about the whole situation. I even told several friends on what to do but none have been able to understand me. I've been overthinking it everyday because I'm scared it may happen again. My girlfriend told me she wouldn't let it happen again and even stopped talking to her friend in hopes that it'll help our relationship and my anxiety. I'm doing my best to let it go but it keeps haunting at me. I talked to my girlfriend about it and she was very scared and worried, I could tell it bothered her. But she has been telling me how horrified and scared she was during the whole situation. I'm doing my best to trust her but I'm still a little scared because of it all happening. Our relationship has been growing stronger because she hasn't been talking to her friend at all.

My anxiety has been better but like I said, I overthink about the situation. And it continues to play inside my head like it happened yesterday. I'm absolutely terrified...and I want to forget about it but I'm having a hard time. If you can help me on what I should do I would appreciate it so much.

Thank you for reading and I hope you all have a better day.