<human>: Can you provide a brief rundown of the text in within 3 sentences?

I will try to keep this as simple as possible

I (23 M) just ended a one year relationship with my SO (20F). I had no idea that our relationship was in jepordy until she dropped the bombshell on Friday that she had been basically flirting with another guy to the point of sending nude pics/video to him. She had been doing so for a short time, apparently. She told me that she simply didn't care anymore. Shes been feeling that way for awhile, and she kept her feelings bottled up until she burst. 

She got drunk at a wild party and slept with another guy (not the same one as aforementioned) this weekend before our relationship "officially" ended. Reason being, It was over in her mind but we had never agreed that it was over, so i assumed that we were still dating..

I'm not 100% innocent: She had caught me a few times talking with other girls in the past. It wasn't in anyway cheating, but it was enough that I tainted her trust. We worked things out (or at least I thought we did) and I stopped the BS because I didn't want to lose her. I know for a fact that what I did eventually lead up to the end of our relationship.

 I know I fucked up. Shes had her trust broken (badly) before we met and she was already so sensitive since the beginning that I should have known that she would take these things very hard.

I don't know if what she did was actually cheating because of the situation, and I feel that everything is my fault... is it?
<bot>: GF has a change of heart, doesn't tell me. She gets drunk and messes around with other guys before we officially end it. I'm not 100% innocent either. Is it my fault?