Welcome to HW! I believe you have found the right place, since there are so many nice and compassionate members around here.
Relationships going bad can be one of the most depressing things. You are not the only one who is sad because of that, lots of us are. As I read your story, I noticed you are in a string of bad events happening now. When your thinking becomes negative, most of the things will also become negative. And negativity is something that will spread to others.
You mention your gf has changed, but it could be just you. A negative mood will have you look at things from a different perspective. She could be acting normal, but because you are having negative thoughts you experience it as negative. It could also be that she notices you being different, and thus act different. I am not saying this to blame you, for depression isn't your fault. In fact, it is encouragement, since you can't change her, but you can change yourself.
One thing that struck me in your story is the last sentence, where you wish she would be like she was at first. Maybe you could see that from a different perspective as well. Creating an image of how some should be like can only generate negativity, because they never will be as we want them to be. Rather than to create an image of how he or she should be like, we can accept that they are who they are. If we are able to drop our images of people, only then we can realize that everyone is perfect in their own way already. Expectations will lead to disappointment at some point. So maybe it is possible that you can accept your gf just for what she is, not for what you want her to be.
I dare to push this theory even further by saying you can't fully love someone unless you totally accept them, and thus dropping the "perfect image". This way, you will lose your attachment to someone. I believe that attachment is negative, since you believe that certain things are "yours". You will feel bad when you lose them. On the other hand, love is a fully positive emotion. But love and attachment tend to go hand in hand. We love someone, and thus we become afraid of losing them. Which is a paradox, because love based on attachment will be love based on the fear of loss, and thus it won't be love for that person, but love for yourself. You don't want to lose someone or something because it will make you feel bad. True love however will be based on the person in question rather than yourself. True love will be that all that matters is the feeling of that person, not yours. So it can't be based on attachment.
Ones again, I am saying those things not to hurt you or make you think you are "wrong", it is just my point of view. Things like this helped me a lot to get over my depression. I hope you can find something of use as well!
All the best Zarkencole!
Acceptance is the key
The World is but a reflection. Smile, and it will smile back.
Try to keep smiling! :)