I'm going to tell my girlfriend she can't use depression as an excuse
Can you explain why you would say that to her?
It's not fair to blame others just because you're depressed.
I agree that they should get help for their mental illness instead of using it as an excuse. Hopefully you're both able to make it through this.
She thinks I did this to her.
I think you are right to be annoyed that she is blaming you for her depression, and this sounds like something that you should speak about with her. You two can make it through this with some good conversation.