I want my significant other to care as much about her fitness and appearance as I do
What is it about her that you are trying to change?
I just want her to be more fit and attractive.
If you want a partner who is more into fitness than your girlfriend is, then perhaps you are not suited for each other. If you find yourself trying to change your partner to be what you want rather than loving her for who she is, then your relationship is probably not going to work out.
I just want her to pay attention and not just keep on eating and being lazy. Like, work out with me?
Have you asked her to join you on a workout? It's wrong to try and criticize someone over their appearance. If that is the only reason you fell for them, then you probably are not a good match.