I have horses and i also have a boyfriend. He comes down to help me (even if he doesn't do much!) Why dont you try to help her with hers then maybe she'll get the work done quicker meaning you get more time together?!
Sometimes we must face up to realities. If your girlfriend puts her interest in horses before you, a choice must be made. Put up with it or find someone else. I am not being nasty about this, honest, its just the way things are sometimes. :o)