My girlfriends depression has progressively gotten worse- I want to stand by her because I love her but she's not the same person anymore what do I do?

What to do? Standing with her means be loving, supportive, making sure she takes her medication on daily basis, seeing her physician and her therapist and making her exercise, since exercise, can increase endorphins and can be helpful in decreasing depressive symptoms.
Seek help. Seek professional help including psychiatrist for medications, counselor to explore issues and teach coping skills and a life coach to teach proactive living skills and explore measures to improve yourself.