Did you tell your girlfriend that you have a passion for long hair??
When I met my wife she had short hair. After a year I told her that I would love to see her letting her hair grow long. I confessed to her that I have a passion for long hair and that it sometimes gets me exite. To my surprise she wasn't mad at me and was glad I told her. She starts pointing long haired women to me to see how I react. We're still happy married and she even hasn't a problem with my ponytailcollection (yes my passion goes that far that even I have a gorgious long haired wife I love to collect real long hair).
So my advice: tell her that you have a passion for long hair but that you only love her!!