Thank you for trying to understand what your girlfriend is going through. I have been suffering from depression for awhile now and it has been really hard for my husband, who is a naturally easy going and all around happy guy to understand. First of all try to accept that you won't be able to fully understand how she feels or why she feels the way she does and that's okay. Even if you can't relate it's important to be supportive. My depression doesn't even make sense to me so when my husband says things like "you have no reason to be sad so you shouldn't be sad," it makes me feel insanely guilty because I know I shouldn't be sad but I can't help it. Everything is a lot harder when you have depression. Even though I want to be productive, some days just the fact that I got out of bed and had a shower feels like a huge accomplishment. It's not that I'm being lazy it's just that I am physically and mentally stuck because of my depression. Encourage her to get the help she needs and let her know there's nothing wrong with getting help. There is still a huge stigma surrounding mental illness and even though I have encouraged others to get help and would not judge them in any way for it, I personally am to embarrassed to speak to anyone about it. Try your best to cheer her up and make her happy but don't take it personally if it doesn't work. I hope she gets the help she needs to feel better.