I was having trouble with my girlfriend and she told me it's because I wasn't sure I wanted to get engaged. She cheated on me with at least four guys. Maybe more.

How do I know this? I saw her Facebook. Tons and tons of proof that she never loved me and she was even laughing with one guy that when I proposed she was gonna say no.

I haven't told her I know this. I don't know what to say but I am done.

EDIT....I'm gonna follow everyone advice and just ignore her