How to get my girlfriend to speak Spanish with me?

My girlfriend is from Mexico. I am from the United States (white). We have been together for six years, and plan to get married. We have had this problem from the start pretty much. She does not want to speak Spanish with me EVER. Most of the time I just try to forget about it, but it is hard for me to do. I love the language, so much so that I am going to major in it when I return to school. I notice time and again that the only times she will speak Spanish with me, are the times that she is practicing for someone or something else. She is not extremely confident in her own abilities, which she has said is part of the reason for her not speaking it with me, but to me it seems that it should be all the more reason to do so. If she only practices once or twice a year for special occasions (seeing family, for a job), she is never going to increase her fluency. Here I am, ready to go any time of the day, and she wants nothing to do with it. What hurts me the most is that it has to be FOR OTHER PEOPLE for her to even speak Spanish with me. It's like a slap in the face to me. She says that it is nessesary for her to practice or use Spanish for her family, because they don't speak English, but because I can speak English, she sees it more like a hobby of mine, and therefore not as important. Also, another reason she says that it is not so important that I learn Spanish is because I am white. It is not part of my culture or heritage. I thought we had moved past the race or nationality issue, but I am reminded every time this subject is brought up that, at least in this area, we are not. We have spoken about this many times throughout our time together, and the problem has never been solved. I have exhausted all of the angles I have to come at this one. I need your help. Please advise me on what to do. Thank you.