This could be long. A bit of a warning.
TL;DR I M20 posted a couple months ago about me and my GF F20 arguing. She said she was late to work, but it turns out she had gone to the lake with her girlfriend and her girlfriends boyfriend. However, something felt off and I told her how I felt and it turned out she was lying to me and had decided to skip work and go to the lake with her guy friend. While I hadn't met this guy friend, he was very new to my girlfriend and me as she began talking to him a couple month into our relationship. Anyway, this guyfriend became very close to my girlfriend and began driving her places and inviting her over to his house after she got off. He also admitted to having feelings for her a few times, but that he wanted her "as a friend". After this, I began questioning this guy. I only heard about him from what my GF told me about him, but everytime she'd hang out with him and she'd snapchat me, he wanted to speak to me and he'd ignore me completely, which further felt very odd. She was also very short when she would be on the phone with me around him.
Fast forward a month after that lake incident with this guy, he invited her to his house and she obliged and decided to go there with him late at night. She told me he was being very close and personal with him, but she kind of liked it. This disturbed me very much so. He then invited her to the club with him alone and apparently he kissed her a few times but my GF told me it "wasn't actually him" as he was drunk, but I felt otherwise. She said she'd cut contact with him though after that as she "felt she was getting too close to him and wanted to step back". Anyway, a few weeks after that and he invited her to a Christmas event, she accepted and went. I told her it made me feel uncomfortable and reminded her that she told me she wasn't going to hang out with him anymore and she asked me if I trust her. She ended up going, and it made me so uncomfortable. I was irritated, but it's like I've almost become numb to it like if I say something, everything will go down, so I just went with the flow. A few weeks passed and everything was going well between us. I managed to throw away my irritation and look to the positives, but everything that has happened has lingered with me. Last week, she posted a snap saying "Thank you babe 😘 ❤️", holding a coffee. I learned about this from a family member who's always friends with her on snap. She saw the post and asked me, "Are you with your girlfriend? She posted this snap." I went to snap to find the post and asked my GF about it. I asked her why she posted that if I didn't get her a coffee, and she told me it wasn't directed towards me. I asked her who it was directed towards and she asked me if I trust her and how it seems like I don't trust her. I told her I was simply curious who she directed that to. She told me it didn't matter but then told me she refers to her friends as "babe" often. I was like, "You know what, that makes sense. Some women do refer to their friends as babe".
That night, we were talking it out and she told me she slept with another guy friend before we were official and asked if that mattered to me. I told her it didn't matter as we weren't actually exclusive yet. It didn't match up though. That night she referred to when she slept with the guy friend, she said his girlfriend was there with him in the same apartment. I asked her about that and she said they broke up when she was there with him and they did it after the break up. Having been lied to before by her, I tried to believe her, but I feel so disturbed about this as the stories don't match up and she has shown to lie about this stuff before. If she was the other one in that situation, who's to say she's not cheating on me. She has told me that she cheated on her ex out of anger that he cheated on her, but she told me that she was stupid for that. I thought it was over after that, but today she sent me an sms message saying "You can come over I will wait for you 😘😉". This once again started the cycle again. I asked her if this was meant for me and she said it wasn't. I asked her who it was from and she started sending me messages about our sex life. I figured she was trying to change the subject. I asked her once again who it was meant for and she said it didn't matter. I told her it did matter and that if she has feelings for someone, just let me know so I can move on. I told her it was flirtatious and she denied that. She's now sending me messages telling me everything was a mistake with me and she shouldn't have been honest about anything.
I really don't know what to do. I feel like an insecure piece of shit for this, but at the same time, my trust for her is definitely damaged and for me that's hard to gain back.