Help advice needed

I want to make my gf happy. But it seems impossible I've been with her for years and I love her to death. But she acts like she can't stand me anymore. I say one thing and she just snaps on me. She's been constantly pushing me away. She says she loves me and that nothings wrong. But I can clearly see something's wrong. Some days we get along fine. Others not so well. What can I do to put some spark back into our relationship. I don't want to lose this girl