This problem manifested itself in January 2017, when my friend told me that my (22M) GF (22F), who was then my hook up (had caught feelings for), was hooking up with someone else back home since October. She rejected me for him on Valentines day. I didn't want to know this information, but since me and now GF were just FWB back in October 2016, I realised I had no reason to be jealous or upset. Still I found it troubling that I knew some things about her personal life that hadn't come directly from her, and that she didn't know I was aware of. Also, that she strung me along when we first met. I spoke to her today and told her that it was bothering me, and that I knew that she had been dishonest to me. I confronted her and made her tell me all about it. I found out about everything, fuck me - it was a bad idea but i said screw it, might as well inquire about her sexual past and understand her character. They used each other for casual sex, did everything except anal and she happily obliged. She hates him now, but the story makes me feel shitty and her reputation for this also makes me look bad.

I also had sex with a few girls in between since we stopped talking after being rejected on Valentines day, when NC and since August, she came back to me and we have been banging like rabbits. She says I'm the only one who has ever made her cum. Today i confronted her about what she did and told her i needed time. She said she would do everything to be with me. I told her im going to fuck other girls to feel better. So how should i go about this?

TLDR; GF was having some hot steamy sex with another older dude at the beginning of when we met (we weren't really dating but she knew i liked her), and i found out through her friends. Confronted her today, and told her i needed time to think about all this after fucking her a few times and having emotional talk.