Uhm ... well.
Here's the thing.
If she's your housemate's girlfriend, using magic to coerce her into having sex with you behind his back is just begging the Universe to club you upside the head with a karma-baton.
There's nothing wrong with being attracted to someone who's unavailable to you, since, for the most part, that's not something you can control. You CAN control the dignity and maturity with which you approach the situation, and what I described above is the very opposite of both.
If you like, you can search for spells intended to break them apart or make her attracted to you. Neither are very noble endeavors either, but at least they don't involve forcing your way into her ladybits magically.
Ultimately, my advice is this: leave that sleeping dragon well enough alone. There are plenty of other available women who would be happy to oblige you with their company. You should focus your efforts, conventional and magical, toward attracting someone less 'off-limits.'