Rhonda Byrnes isn't an occultist by any means, she's just a clever salesman catering to the vain entitlemen of middle class people. Fuck that.
You seem to be insisting on using sorcery - whether that be spirit work, spell casting, energy manipulation or psychic warfare - in order to achieve your goal. And yet, you haven't considered the strategic implications, something that is absolutely crucial in this kind of work, whatever your goals may be.
She might get a heartbreak and ferociously decline all your advances. She might harbor unresolved feelings for her ex and leave you. She might come to the conclusion that she "didn't knew what she was thinking" when she agreed to the relationship. You might harbor a subconscious guilt that reaches a boiling point over time, and when you tell her the truth, she reacts with disgust and leaves. Because it wasn't her decision, she will not have the fortitude to stick around with you and thus be persuaded by her friends to find someone else. etc, etc, etc.
All these risks can be avoided by simply following Shinichi's excellent advice. Hence, strategically speaking, not adding the ethical aspects into the equation, that is the best thing to do.
Ethically speaking, she's a human being for fucks sake, not a doll of whatever romantic fantasies you have built that can easily be molded to fit your fancies. That is not love, that is possession.
Fun fact! The word "persona" comes from a latin root signifying a theatrical mask. Stalking her on social media, eavesdropping in on conversations, analyzing her behaviour by observation, even talking to her in public, does not equal "knowing a person well". Unless you have an honest and trusting relation with a person as a friend, you hardly know them at all. Wife my ass.
Don't be an entitled little brat - she doesn't owe you anything. And she is certainly not responsible for your happiness. Nobody is but yourself. She is not the graceful saviour of your misery, irregardless of what Hollywood and social conditioning have made you believe. You should be able to live a fullfilling life just fine without her, and if you can't, you aren't emotionally mature to engage in a healthy relationship to begin with.
You have not considered that the guy she's with is actually her "cake guy", who took action while you were busy fiddling around in the bushes. You have not stepped into his shoes and asked yourself how it would feel if some sleezebag came along and broke your relationship. You have not stepped into her shoes either and asked yourself what she wants, and how that possibly might not be you.
Besides, why bother throwing black magic at her at all - she already has the curse of being a pretty girl. Give her a break.