Well, you're really having trouble with this. Think about what you have a passion for, besides this girl, and try to think of ways to occupy yourself with that. My mind is always on women as well. I can't help that, I'm a man. You can't help that you're infatuated with this girl either. But if you don't try to use your other passions, whether it be on a subject you like, or a hobby, a past-time, etc., then you're not going to be able to get away from this.
Like i said before, convince yourself that you don't need to think about her, and use whatever you can think of to achieve this. In reality, you really do not need to be thinking of her all the time. Since you can't get married to her for some time, this will have to be the case.
Now, this last resort, which may or may not work with your family and theirs, seems to be the safest method. I have seen other guys I know get themselves engaged to a girl and remain engaged for some years, until they become settled in life and then they finally get married. This ensures that you have her to yourself, and that you don't need to worry about anyone else having her. Perhaps you fantasize so much about her because she isn't yours yet. I don't know if that makes sense, but this is what I surmise. Usually, when people see something they can't have, they really want it. But, when they actually have it, they don't need to think too much about it since they don't have to seek it out. I think it works the same way for men and women who like each other.
If you haven't tried getting engaged to her already, and staying that way for some time, then I suggest you bring it up with your parents. It might relieve your mind.