Advice please

Toward the being of the I met this frat guy and I was fooling around with him and I almost had sex with him again last night. I have asked him on several occasions is he had a girlfriend because I've seen him with a her at a few different sport events but he always told me she was just a friend nothing more.... But I found he lied to me about not having a girlfriend, he's had a girlfriend this whole fucking time!!!! While at Waffle House I was scrolling through twitter and seen where she had tweeted about him being the best boyfriend. I felt sick, I couldn't even eat my food but I sent her screenshots of our conversation. I felt so sick, ashamed and disgusted of myself for believing him. I still feel absolutely horrible about the whole situation and that's what I told his girlfriend when I sent her the messages. I wish that I could take it all back and change it but I can't. She asked me to meet her this Friday so that I can show her the messages in person, and prove that I didn't fake the screenshots. But he's not gonna feel any remorse or anything like that, those feelings fell on me.... Unfortunately he's been lying to me and I fell for it so I look like the slut not him.