re: Interested to hear your story if you found out your partner cheats with multiple people

She has a bad mental problem. Sex addict. Addicted to a cheating lifestyle. She is not curable. This is a very sad story, but, you can't allow yourself to be a part of this story any longer, unless you are also mentally disturbed somehow.

Break up with her on the sensible grounds that she is not ready for marriage. Tell her that in a calm manner. No need to get emotional about it.