I was in a similar spot. Girlfriend has almost waist length hair but reluctant to grow any longer, but her hair quality is amazing and it's thicker/softer/silkier than any girl I've been with. In my opinion you have two options:

1) if you have an amazing connection with her and love her, accept her as she is. Waist length is still longer than the average woman. Even maintaining that length can be a chore, and she could already think it's a lot to ask of her, but I assume she would do so because she loves you, too, and understands the importance of your fetish.

2) break up with her in hopes of finding a rapunzel out in the wild, or a girl who would be willing to grow your hair super long for you. Both of which, I think are harder than you would think to find, especially if you pair that with an amazing emotional connection.

Personally, I chose option 1. Though my gf's hair is not as long as I would ideally like, she incorporates her hair in every aspect of our sex life and it still drives me crazy. That being said, when I see a woman in public with gorgeous long hair, my eyes will wander, but it's not enough to make me question my relationship.

Also, I've found that laying off of the rapunzel porn has helped me appreciate shorter (but still gorgeous) hair a lotttttt more.

Sorry for the novel but hope this helps. Cheers!