Hi,
I would like to try to offer some advice from two different perspectives. First, as a long term caregiver who recently lost my loved one, and second, as a person with a disability who can relate to some of the feelings your girlfriend may be experiencing.
First, to repeat what Ashley said above, kudos to you for reaching out and sticking by your girlfriend. I don't what stage you are in your relationship, but the sincerity of your post shows that you are willing to stay in for the long haul. Caregiving for a loved one will present unlimited opportunities to bond deeply with each other. Take every advantage of those opportunities. There will be some resistance on her part (and maybe on yours) in the beginning, but be patient, be understanding, and most importantly, just be there for her.
One thing that many people don't seem to understand about dealing with those with disabilities is that you must let them have every opportunity to do things themselves. To me, nothing is more frustrating than for someone to "take control" and do something for me that I can do for myself. It might be more difficult, and take a little longer, but I'll figure it out, or I'll ask for help. As your girlfriend gradually loses some of her abilities, she will have difficulties doing simple everyday tasks, and it will be frustrating for her. Whenever possible, let her figure out how to do it, and be her backup if she can't. I think this is the origin of the latest politically correct term for those with disabilities: Differently Abled. A large part of adjusting to a disability is learning how to use the abilities you still have to compensate for and overcome the abilities you are losing. Unless your girlfriend is placing herself or others in danger, let her figure things out on her own, but show her that you are there if she needs you.
Good luck to you both,