bottom line is motivation has to come from within. you cannot force people to do it, that will only push them away. if you want to workout with your gf, then find a buddy to work out with who atleast is close to your level. why workout with a gf who is going to slow you down?
if you are unhappy with her being out of shape or fat.. then go find a girl who is more into fitness. I did date a girl who became a fitness freak after she met me. went from never setting foot in a gym and eating mcdonalds. to becoming a vegetarian and being a gym rat. i didn't force her into it, it just clicked for her after she met me and she learned a lot from me and started to do her own reading on fitness.
she literally never played a sport in her life.. the first time i took her to the gym she was freaked out. now she fucking owns the gym and everyone bows to her. so leave her alone man, she will find her way