As a true bro, I'm going to give you the hard truth...
1. If she's crazy enough that you're worried what she'll do AFTER you break up with her, you don't need that shit. Cut your losses.
2. As for you worrying that you will develop "oneitis"? It sounds like you already have because you don't want to lose her. That is the definition of oneitis.
But, since that's not what you want to hear, and you're just going to ignore all that, here's what you really want:
Jedi Fucking Mind Tricks:
Tell her you're not "ready" yet. Make it seem like you might be ready in the near future (which is never), and that you want something "deeper" with her. And then, once she trusts you and thinks that you two are going to be together forever and ever... tell her that you want a 3some with her best friend. She'll be repulsed (unless she's crazy), and she'll actually do the break up part for you. Then, in her mind, it'll be her fault because SHE broke up with YOU.
And then Dr. Ripley prescribes you fuck 10 girls and call me in the morning. You're welcome.
