I want to leave her and want to teach her that she will never find anyone like me in her life
That's a lousy reason to do pretty much anything.
I'd say you're pretty overconfident for someone who's been cheated on a half a dozen times. She has
found someone who she thinks is better than you. Sorry, I know that's harsh, but she has. So rather than sticking with her to somehow try to teach her a lesson (and believe me, she is clearly not receptive to this), leave anyway - for your own reasons.
As for your friend who she cheated on you with? I'd say this is not such a great friend, eh? You might wanna cut off all contact with him, too.
As for you
- you need to learn about yourself, why you would willingly want to spend time, and even think about marrying, a person who so obviously and thoroughly disrespects you and treats you like crap.