Why does it hurt so much?

Ok, some will know I decided to end an online relationship with a girl this week (in [URL="http://www.loveforum.net/love-advice-forum/40153-choosing-sucks.html"]this thread[/URL]), I managed to have the deal so we are friends and I'm free to have whoever I want, while she has her boyfriend and the freedom he gives her to have sex with the people she wants to.

We are both pretty touched about this, but it's for the best imo, although she has been crying all week and has started smoking (after 4 years without smoking), I didn't think it would affect her so bad. She said she is mostly hurt because she thinks it's her fault I was mislead because I didn't know about her boyfriend for a while, and she didn't want to hide it from me. I don't blame her for it though, I kept on after knowing about him, so it's my fault.

She updated her facebook "relationship status" to "in a relationship" with the boyfriend on thursday and I just saw it... It hurts like if someone just ripped my chest open and took out my heart. Or worse.

Why the hell does it hurt so much? I still have her as a friend and I knew about them for about a year, it shouldn't be this way...