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Now, instead of hating me and apparently wanting to kill me, what would you do in a situation like this? Better yet, is there anything you could do to keep her from doing this to another guy?
This is a PUA forum. You don't seem to have read up on any of it. Pretty much all of the science can be summed up in the phrase Nothing is a big deal.
I cannot believe you think a major outburst of cruelty is doing you any good in this situation at all. I'm not being sympathetic to the girl here, the very opposite. With your emotional outburst you show her that you really, really care about her and are really, really hurt by her. Those are big rewards for her whoring. Hell many women would just cheat on their man just to get that reaction from him.
In every case I can think of where a woman behaved as you did, I knew for absolute certain the girl was passionately in love and lust with me. I knew I owned her at that point.
What you should do:
Just let her do her speech, smile, mention something briefly about the relationship coming to a natural end and how you had a really great time together and then move on. Maybe thank her, as you had a real connection with another girl recently and you wanted to pursue that.
DO NOT REWARD HER WITH ANY KIND OF EMOTIONAL REACTION AT ANY TIME.
Then cut her dead, don't communicate with her again. If she talks to you be polite but always have something else to do.
Silence is a far more effective a weapon than an insult. It might be worse to be hated than loved but it is much worse than either to be simply ignored.
As to how you can stop her cheating again, you can't. Women (and men) are hard-wired to screw around, live with it.
Unfortunatly the rules of game, and the rules of relationships differ.
Not caring, really isn't much of an option in a relationship scenario for practical but also emotional reasons.