There can be many reasons for "why your girlfriend never appreciates you"?
This reason may break your heart but 99 times out of 100, this will be the correct reason.
"She does not care for you anymore. She is done with you, and found someone else."
It's better you move on with your life!!
Just like any other concern in a relationship, you simply need to be open and tell her that you expect more appreciation. There could be several reasons that you may not know, she could be busy with her career, worried about her parents/family/friends. Or she might have become comfortable enough and in a place where appreciation is not a big deal in your relationship. It is obvious because as a relationship matures with time, people do tend to take it easy. So, you have got to talk! I mean men and women are different for a reason. And its silly to expect anyone to understand your problem telepathically. Have a healthy open conversation. For better or for worse, talk! :)