You want a spell to make her be the bad guy in the downfall of your relationship, instead of telling her the truth?
Magick can do much, but it cannot completely change someone. It can help you if you are willing to work alongside it, and it can help if you are already so inclined. But for it to force someone to cheat or leave someone is a difficult request at best.
I don't think you will find anything on here that could help your situation. Honesty is the best policy, all I can say is that you may be able to find something to make it easier on everyone in the situation.