Confusion

I have so many mixed feelings towards everything. She seems okay with us breaking up. It seems like I'm having a more difficult time dealing with this than she is. She's probably going out; drinking, making out and having all the sex she wants. I still have trouble getting over it. I don't feel like I will ever find a relationship quite as great as ours. Yeah, she found this guy attractive and she had sex with him and he got what he wanted. She rewarded him for him being a douchebag and hitting on her when she had a boyfriend, but honestly, I watched the video on youtube and she was practically mesmerized by him. She was constantly following him with the camera and maybe she did kind of bring it upon herself. I've been going out for the last few nights and for some reason, I couldn't get myself to converse; I was still caught up on her. I don't understand how she could go out and let a guy touch her like that.. in such an obscene and savage way and be okay with it. I mean.. he was inside her for godsakes. How is that okay? I guess I will never quite understand. I sent her a care package while we were still trying to work things out, I'm pretty sure she just threw out those pictures of us as if it was nothing. Why do I care so much? Why can't I get over this? SHE FUCKED A GUY BEHIND MY BACK TWICE. Why can't I respect myself enough to stop talking to her?