So my girlfriend has pretty sever depression and her parents won’t let her get diagnosed so she doesn’t really have any official help for background. I also have sever depression but I am diagnosed

So during the day my girlfriend is all good and happy and very loving and it’s amazing, but pretty much as soon as nightfall hits she changes, she’ll go from taking about getting married one to getting really depressed and she  starts talking about how she’s not right for me, screws everything up, a bad person, doesn’t know how to be in a relationship. And a whole lot of stuff just like that, and I really try my best to comfort her when this happens and reassure her that she is right for me and that I don’t care if she’s screwed anything up because I love her and other things along those lines. But it just never seems to work, at this point I just don’t know how to help her. Please any help is appreciated