You can't. Seriously, you cannot.
As a carer, you will hear this a million times, but trust me, it's true: You can't help someone who doesn't want to help themselves.
Or, maybe she's said she wants to be better? Well, words and actions are two very different things - she needs to start showing you that she is making an effort, and then yes, you can be there to support her, but at the end of the day, you are not responsible for making any of that happen - she is.
She needs a good GP, a psychologist, and a psychiatrist. She needs to open the damn curtains and let some sunshine in, and eat properly, and get some exercise. I understand that the depression side of things can make all of these things difficult, but you can't give her vitamin D injections, or force-feed her fruit, or make her talk to doctors when she doesn't want to. It just won't happen.
I hope that doesn't sound too harsh, it's just that it's the truth. I tried for a while to 'help' my BP boyfriend, with no results, and looking back, I can see now that it's because he wasn't willing to put the effort in himself.
Please don't misunderstand - I am not saying you should leave her to her own devices. By all means show your love and support - it's just that at the end of the day, the things you both know she needs to be doing, are just that - things SHE needs to be doing.
Good luck with everything, it is not an easy path to go down. You sound like a very caring person and I hope your girlfriend is able to see that. 
Cheers,
K