And you getting worked up about it isn't likely to make things any better.
If she's on them, she needs them, and the doctor will have most likely suggested her going on them in conjunction with a course of therapy, which works for some people and doesn't for others. If you hadn't noticed any major problems with her mood up till now, they're clearly working and you'd be a dick to mess around with them.
You can't 'fix' her. You might want to. By trying to force her to come off them you run the risk of making her much unhappier and as if she is not in control of her own life, and it will only make her problems worse. This is her illness, and she will get better when she gets better. You pressuring her to get better is going to do **** all to help.
Sorry if this sounds brash, but you really need to think more about what your girlfriend needs and less about your own personal preconceptions.