Not knowing anything about where you are at w/ your stats, it sounds like having a better grasp of food quality and portion control would go a long way for you.
People usually have no sense of how much or what they are really eating. I would suggest you write out a typical day's meal plan (or just pick a day & record what you eat) and then put it into a food counts program like
FitDay - Free Weight Loss and Diet Journal. I like this program - it is free, online and very easy to use. You enter your foods and the portion size and it will calculate your total calories for that day as well as the macronutrient breakdown (% protein / % fat / % carbs).
The next step is to get a sense of how big portions are and the true calories associated with them. General guidance is to stick to simple foods (i.e. minimal processing, keep the sauces, deep fry, etc. to a minimum) to avoid the excess fats / carbs. And also get a sense of portions - e.g. a good serving size for protein is a portion the size of your fist. For ex, I'm female - my typical protein serving size is about 35 grams = 4-5 oz chicken = 4 oz steak = 6 oz fish - these portion sizes I can eyeball as being about the size of my fist. If you're a guy, I assume you're going to have bigger fists so your portions are going to be more like 6-8 oz where mine are more like 4-6 oz, which would equal around 40- 50 g protein.
Hope this helps. If you have some basic guidance on quantifying what you're eating, it helps to keep your overall consumption undre control.
I would also then recommend you check out Built's "Homework 1" (I'm sure she'll chime in soon enough on this thread - check her signature for the link). This will also help you to determine how much you should be eating for your size, which, in turn will give you an idea of how much more or less you are actually eating typically.
It is annoying as hell if you've got someone nagging you about what you eat. I grew up w/ that and have spent a large part of my life developing a better relationship w/ food. My response to my mom getting on about what I ate growing up, was to wait until everyone went to bed. I became master of sneaking food out of the kitchen. I still have hangups up about it, but at least now I have something logical to help track what Im' eating instead of just being driven by whatever sounds good at the time. I'm sure your gf is concerned about your health and wants to help - but its really something YOU need to take responsibility for if you both acknowledge that it is an issue.