I don't desire sex with my girlfriend anymore, any advice?

Interesting question. I understand what you mean though I haven't ever got to that point myself. So I'm not sure I will give sound advice that I can prove works. Though with any relationship communication is a huge key to it. Tell her how you feel and see if she will be interested in trying new things like the stocking and heels, or role playing. Maybe if she understands where your coming from and that you care about your relationship she will be more interested in giving things a chance. Good luck