How can I help/motivate my girlfriend?

Communication. You may or may not help her motivate herself (in most cases it's a "not"), but talking will help resolve any resentments starting to build up. Ask her to be really really honest, and assure her you are 100% behind any choice that will make her happy, but as her: do you really want to do this? If her answer is yes, listen to her. See why she is having a hard time.

She may not know it, but she doesn't have to do exactly what you're doing to get results. She's her own person and can try other ways that may feel more sustainable to her. Remind her that any progress is good progress and that men tend to make faster progress simply because of their physical build. Even some women tend to progress faster than others because of various reasons. Any speed of progress is good progress (took me 7 years to lose 140 lb).

If she is still interested, maybe direct her to the forums to ask her questions if she has any. Maybe she won't feel left behind if she sees how differently people progress and how differently they approach weight management and fitness.