Carleton, and others, have you checked out this site:
http://eatingacademy.com/nutrition/k...d-state-part-i
This guy does a ton of self experimentation, and is really well educated and researched in terms of nutrition and ketosis. An interesting thing he mentioned is that keto and other low carb diets are actually more beneficially for individuals who start out with insulin dependence, statistically. (There was a really good study done comparing low fat, and low carb diets, and basically they found that the responses of individuals to the diets were pretty well correlated with beginning insulin resistance, and other markers. It's far from conclusive, and Stanford is currently doing a 3 year clinical trial to shed more light on this. [aside: I just signed up to take that PI's nutrition class next quarter! Psyched])
Anyway, doesn't sound like insulin resistance would make keto more challenging. One thing, is you probably eat much more protein than your GF? It seems like to get into Keto, (efficiently/quickly) you have to kind of limit your protein intake too. Excess protein can be converted to glycogen in the liver, which reduces the need for ketone bodies. According the guy I linked to above, the higher your caloric need, the higher percentage of fat you need. I'm assuming you need more calories than your tiny girlfriend, so actually eating the same macros as her (aka the same foods just larger portions) wouldn't work. So maybe it's not that strange?