Hello and welcome to HealingWell,
I am sorry to hear your significant other is having problems with depression. If you want to maintain this relationship, I think that perhaps the best thing for you to do is to learn to love the person your girlfriend is *while she is depressed*, instead of wishing for better days so that things can get back to "normal". You already love who she is when she is not depressed: love the depressed person too, and you will be able to love the whole person.
You may want to look into counseling for yourself as you need to take care of you as well as be there to support your girlfriend.
Here is the link to a site that has some great tips on supporting a family member who has depression: