Nothing destroys a relationship quite as well as making one of them obsessed with someone else.
My first boyfriend cheated left and right, leaving me for a “just a friend” side chick (she was literal trailer trash and notoriously promiscuous…but she figured out she could lure him away by plying him with drugs).
First, I did a binding on her (yup, just like in The Craft…except I didn’t bind her from harming herself).
Then, I did a witches’ ladder spell on her, specifying the many ways in which she would be punished and unable to escape said punishment.
And then I brought out the big guns.
It works, BUT you need to give the universe time to do its thing. It took several months, start to finish, but he crawled back (repeatedly, for years) and his side chick’s life was ruined.
You can do this. Just be patient.