I say look for a new girlfriend instead of losing weight. If she can't take you in your condition right now, then what will she do when you're 58, fat, bald, and unable to bend down to tie your shoes? Would she just leave you?
Talk it out with her first. Ask why she doesn't accept you in your current condition. If she is serious about fitness, then be serious about her fitness as well. Perhaps both of you should join a gym or something and perhaps both of you can realize at one point and time, that perhaps it isn't always about how much you weigh.
If she can't make a commitment to you, then why should you make a commitment with her?