Sounds like she knows what she is doing, how it hurts you but keeps on with her behaviour. She is being unfaithful, maybe not in the traditional sense but in the emotional sense. and sounds like there is very little trust left in the relationship.
She is also minipulating you but threatening to hurt herself if you leave. I say kick her ass to the curb. Relationships are hard as it is. And if she persist on the virtual cheating what's to stop her from doing in real life. Trust your gut. This is not healthy. If she loves you why should you beg her to leave this hurtful behavior in her past.
She is the one who has issues to deal. Looking for attention from other men to feel good about herself and feed her ego. She is obviously not mature enough to be in a serious adult relationship.
Go with your gut. Do what's best for you. Sometimes the hardest thing and the right thing are the same.
Tagging @chekohler, @penderis and @miniming
I think looking at the flags I have nothing to say to one side of the story ;)