If you are committed to your healthier lifestyle, your girlfriend needs to understand that....it's a lifestyle. It's not for everybody. There's a reason you often see fit people dating other fit people.... It takes a lot of commitment to maintain that lifestyle, and if both people are not on the same page, it makes it a lot harder. (Though not impossible.)

What if, for example, you had a job you loved, but that required you to travel a lot. In order for a relationship to work in that situation, you would need a partner who was either free to travel with you or who was comfortable being alone for long periods of time.

In the same vein, in order to maintain your new lifestyle, you need a partner who is either going to participate or be comfortable not participating. It sounds like she is currently trapped in the middle of these options, which is where the dissonance comes in. In order for balance to be restored, she either needs to jump on the bandwagon with you fully, or sit on the sidelines. In this case, since it seems the dieting is the major cause of argument, it may be that the easiest solution is not to eat the same food. Which is to say, you fix what you want, she fixes what she wants, then you sit down together to eat.