She plans sex but still says NO, we only do it twice a month now.?

I've been dating someone now for a close to a year now. From our second month, we went from having sex every day to once a week, if lucky. I've tried everything, i've bought toys, i've done oral countlessly and etc... She still doesnt budge... I've reached my wits end at the moment cos i really dont know what to do. I really love this woman but i'm struggling to adjust to the whole once or twice a month thing now. Even when we have it, after she cums, I start feeling like i'm raping her cos all she wants me to do then is finish. This is seriously becoming a problem. At times she'll give the age old excuse that she's tired and that i must wake her up later or in the morning, but still when i do that she still says no. Our sex life has gone from super to non-existant. I've tried speaking to her and she's just elusive about why she doesn't want it or she just keeps quiet and ignores the question. When she's tipsy or drunk, she never wants me to touch her - Is this normal behaviour??? I'm confused, please HELP, What should i do???