Last night I was told by one by one of my friends that my girlfriend of a year and a half had lied to me. I was told she talked to this one dude let’s call him mark. Mark is a 19 year old boy who I’ve hung with a couple times and gone on a two-man with (I was with my gf, he was with my girls best friend). I want to preface this by saying me and my girlfriend had broken up for 2 months becuase I caught her talking to another guy and asked him for a ride to her soccer game in a flirty manner. But in these 2 months we told eachother we could talk to other people but we wouldn’t do anything physical other than maybe kiss. So continued last night I confronted her about this man and she told me she got high with him and got fingered but that lasted like 20 minutes. This was in November, 5 days before we got back together. I was so in love with her and it appeared she was more in love with me. She is genuinely an amazing girlfriend and we made plans to marry but we told each other we’d be honest with each other about what happened during the breakup. She said she was scared of losing me. I broke up with her last night but I’ve kept her unblocked as she asked to give her until summer to “prove herself” it probably won’t happen but she said she would hurt herself. I don’t know what to do. I don’t know why I can’t man up and leave her. What do i do?

P.S I want to say we were both each others first experiences in everything. And I wanted us to be each others last