Assuming this is real, I'd just bring it up in conversation. Like next time she talks about how she hates people with a long hair fetish, you say "What if I had that fetish?" and take it from there.

If she loves you as much as you claim she does, what she probably means is she hates it when strangers are overly sexual about her hair and not you, her BF. Most women can tell these situations apart and if she can't with you, then it wasn't meant to be.

Regardless of what happens though, keeping your fetish a secret with a GF with ankle-length hair is a terrible idea and will only cause more problems the longer it goes on.