I want to tell my girlfriend she doesn't work as hard as me
What makes you think that you work harder than her ?
I've had a look at her work and it is far from being as good as mine.
It's not nice to compare your work to your girlfriends in that way. As long as she is doing the best that she can do than it is fine. You may simply be better at the work, but that does not necessarily mean that you worker"harder" than her.
I want to prove that she's not as good as me.
Maybe she's not good at doing certain work but at the same time you're also not as good as her to do something else. So stop considering yourself superior to your gf.