Dude - leave her.
Twice a month! Holy ****E, seriously? No No, okay. You living WITH her and you are making love twice a month. Something does not make sense.
Well as much as I just said that, because that is of personal opinion I am aware there are people that are very happy to have sex twice per month, as a couple, but then to make it work then that feeling between the two need to be mutual to a point.
I know at least three people currently married I can think about that have both admitted to me having a better sex life than that and speaking to the husband of those couples would think there may be something questionable going on if his own wife is not interested to a degree of what you specified.
Okay, thats about all that I am saying on that. As much as I may want to agree with the statement I quoted above I dont know if I can because it is up to you, if it were up to me then I (at this stage of how I am thinking at least) would not be very happy.